⚡ Add API: SetASDefaultProtocolHandler

This PR adds an API enabling Electron to set itself as the default
protocol handler for a custom porotocl on both oS X and Windows.

For details, please see `docs/app.md`.

Closes #4857

atom/browser/browser_linux.cc

@@ -34,6 +34,10 @@ void Browser::ClearRecentDocuments() {
 void Browser::SetAppUserModelID(const base::string16& name) {
 }
 
+bool Browser::SetAsDefaultProtocolClient(const std::string& protocol) {
+  return false;
+}
+
 std::string Browser::GetExecutableFileVersion() const {
   return brightray::GetApplicationVersion();
 }