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/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(