Bug 1438408 - Shutdown RenderThread if it exists r=nical

gfx/ipc/GPUParent.cpp

@@ -462,7 +462,9 @@ GPUParent::ActorDestroy(ActorDestroyReason aWhy)
   dom::VideoDecoderManagerParent::ShutdownVideoBridge();
   CompositorThreadHolder::Shutdown();
   VRListenerThreadHolder::Shutdown();
-  if (gfxVars::UseWebRender()) {
+  // There is a case that RenderThread exists when gfxVars::UseWebRender() is false.
+  // This could happen when WebRender was fallbacked to compositor.
+  if (wr::RenderThread::Get()) {
     wr::RenderThread::ShutDown();
 
     wr::WebRenderAPI::ShutdownExternalLogHandler();

gfx/thebes/gfxPlatform.cpp

@@ -1052,7 +1052,9 @@ gfxPlatform::ShutdownLayersIPC()
         // This has to happen after shutting down the child protocols.
         layers::CompositorThreadHolder::Shutdown();
         gfx::VRListenerThreadHolder::Shutdown();
-        if (gfxVars::UseWebRender()) {
+        // There is a case that RenderThread exists when gfxVars::UseWebRender() is false.
+        // This could happen when WebRender was fallbacked to compositor.
+        if (wr::RenderThread::Get()) {
           wr::RenderThread::ShutDown();
 
           Preferences::UnregisterCallback(WebRenderDebugPrefChangeCallback, WR_DEBUG_PREF);