Bug 1260243 - Add GeckoThread.waitOnGecko; r=snorp

GeckoThread.waitOnGecko blocks the current thread and waits for previous
events on the Gecko thread to finish executing before continuing. This
is implemented by synchronously running a dummy event on the Gecko
thread. This patch also lets us get rid of sendEventToGeckoSync in
GeckoAppShell.

widget/android/nsAppShell.cpp

@@ -182,6 +182,14 @@ public:
         }
         specConn->SpeculativeConnect(uri, nullptr);
     }
+
+    static void WaitOnGecko()
+    {
+        struct NoOpEvent : nsAppShell::Event {
+            void Run() override {}
+        };
+        nsAppShell::SyncRunEvent(NoOpEvent());
+    }
 };
 
 class GeckoAppShellSupport final