Bug 1515689 - Fix initialization/destruction race in UiCompositorControllerParent r=botond

It looks like we can do initialization and destruction from the UI
thread before the bits that run on the Compositor thread have run. Avoid
this by synchronously waiting on the Compositor.

Differential Revision: https://phabricator.services.mozilla.com/D16596

--HG--
extra : moz-landing-system : lando

gfx/layers/ipc/UiCompositorControllerParent.cpp

@@ -17,6 +17,7 @@
 #include "mozilla/Unused.h"
 
 #include "FrameMetrics.h"
+#include "SynchronousTask.h"
 
 namespace mozilla {
 namespace layers {
@@ -277,9 +278,16 @@ void UiCompositorControllerParent::InitializeForSameProcess() {
   // This function is called by UiCompositorControllerChild in the main thread.
   // So dispatch to the compositor thread to Initialize.
   if (!CompositorThreadHolder::IsInCompositorThread()) {
-    CompositorThreadHolder::Loop()->PostTask(NewRunnableMethod(
-        "layers::UiCompositorControllerParent::InitializeForSameProcess", this,
-        &UiCompositorControllerParent::InitializeForSameProcess));
+    SynchronousTask task(
+        "UiCompositorControllerParent::InitializeForSameProcess");
+
+    CompositorThreadHolder::Loop()->PostTask(NS_NewRunnableFunction(
+        "UiCompositorControllerParent::InitializeForSameProcess", [&]() {
+          AutoCompleteTask complete(&task);
+          InitializeForSameProcess();
+        }));
+
+    task.Wait();
     return;
   }