Refactor as per @zcbenz comments

Also fix issue where we run the single instance callback *not* on the UI thread,
this apparently results in a hung process.

atom/browser/api/atom_api_app.cc

@@ -407,7 +407,12 @@ bool NotificationCallbackWrapper(
     const base::FilePath& cwd) {
   // Make sure the callback is called after app gets ready.
   if (Browser::Get()->is_ready()) {
-    callback.Run(cmd, cwd);
+    // We definitely want to call this callback on the UI thread
+    content::BrowserThread::PostTask(
+      content::BrowserThread::UI,
+      FROM_HERE,
+      base::Bind(base::IgnoreResult(callback), cmd, cwd)
+    );
   } else {
     scoped_refptr<base::SingleThreadTaskRunner> task_runner(
         base::ThreadTaskRunnerHandle::Get());
@@ -522,7 +527,6 @@ App::App(v8::Isolate* isolate) {
           base::ProcessMetrics::CreateCurrentProcessMetrics()));
   app_metrics_[pid] = std::move(process_metric);
   Init(isolate);
-  App::self_ = this;
 }
 
 App::~App() {
@@ -533,12 +537,6 @@ App::~App() {
   content::BrowserChildProcessObserver::Remove(this);
 }
 
-App* App::self_ = nullptr;
-
-App* App::Get() {
-  return App::self_;
-}
-
 void App::OnBeforeQuit(bool* prevent_default) {
   *prevent_default = Emit("before-quit");
 }
@@ -586,7 +584,7 @@ void App::OnFinishLaunching(const base::DictionaryValue& launch_info) {
   Emit("ready", launch_info);
 }
 
-void App::PreMainMessageLoopRun() {
+void App::OnPreMainMessageLoopRun() {
   if (process_singleton_) {
     process_singleton_->OnBrowserReady();
   }

atom/browser/api/atom_api_app.h

@@ -74,8 +74,6 @@ class App : public AtomBrowserClient::Delegate,
   static void BuildPrototype(v8::Isolate* isolate,
                              v8::Local<v8::FunctionTemplate> prototype);
 
-  static App* Get();
-
   // Called when window with disposition needs to be created.
   void OnCreateWindow(
       const GURL& target_url,
@@ -101,7 +99,6 @@ class App : public AtomBrowserClient::Delegate,
  protected:
   explicit App(v8::Isolate* isolate);
   ~App() override;
-  static App* self_;
 
   // BrowserObserver:
   void OnBeforeQuit(bool* prevent_default) override;
@@ -116,6 +113,7 @@ class App : public AtomBrowserClient::Delegate,
   void OnLogin(LoginHandler* login_handler,
                const base::DictionaryValue& request_details) override;
   void OnAccessibilitySupportChanged() override;
+  void OnPreMainMessageLoopRun() override;
 #if defined(OS_MACOSX)
   void OnWillContinueUserActivity(
       bool* prevent_default,

atom/browser/atom_browser_main_parts.cc

@@ -185,7 +185,7 @@ void AtomBrowserMainParts::PreMainMessageLoopRun() {
   Browser::Get()->DidFinishLaunching(*empty_info);
 #endif
 
-  atom::api::App::Get()->PreMainMessageLoopRun();
+  Browser::Get()->PreMainMessageLoopRun();
 }
 
 bool AtomBrowserMainParts::MainMessageLoopRun(int* result_code) {

atom/browser/browser.cc

@@ -171,6 +171,12 @@ void Browser::RequestLogin(
     observer.OnLogin(login_handler, *(request_details.get()));
 }
 
+void Browser::PreMainMessageLoopRun() {
+  for (BrowserObserver& observer : observers_) {
+    observer.OnPreMainMessageLoopRun();
+  }
+}
+
 void Browser::NotifyAndShutdown() {
   if (is_shutdown_)
     return;

atom/browser/browser.h

@@ -224,6 +224,8 @@ class Browser : public WindowListObserver {
   void RequestLogin(LoginHandler* login_handler,
                     std::unique_ptr<base::DictionaryValue> request_details);
 
+  void PreMainMessageLoopRun();
+
   void AddObserver(BrowserObserver* obs) {
     observers_.AddObserver(obs);
   }

atom/browser/browser_observer.h

@@ -55,6 +55,9 @@ class BrowserObserver {
   // The browser's accessibility suppport has changed.
   virtual void OnAccessibilitySupportChanged() {}
 
+  // The app message loop is ready
+  virtual void OnPreMainMessageLoopRun() {}
+
 #if defined(OS_MACOSX)
   // The browser wants to report that an user activity will resume. (macOS only)
   virtual void OnWillContinueUserActivity(

chromium_src/chrome/browser/process_singleton_posix.cc

@@ -1048,10 +1048,9 @@ bool ProcessSingleton::Create() {
   if (listen(sock, 5) < 0)
     NOTREACHED() << "listen failed: " << base::safe_strerror(errno);
 
-  DCHECK(BrowserThread::IsMessageLoopValid(BrowserThread::IO));
   sock_ = sock;
-
-  if (atom::Browser::Get()->is_ready()) {
+  
+  if (BrowserThread::IsMessageLoopValid(BrowserThread::IO)) {
     StartListeningOnSocket();
   } else {
     listen_on_ready_ = true;