Fix shutdown crash in VsyncBridgeChild when the GPU process is enabled. (bug 1314816, r=mattwoodrow)

gfx/ipc/GPUProcessManager.cpp

@@ -397,7 +397,10 @@ GPUProcessManager::DestroyProcess()
   mProcessToken = 0;
   mProcess = nullptr;
   mGPUChild = nullptr;
-  mVsyncBridge = nullptr;
+  if (mVsyncBridge) {
+    mVsyncBridge->Close();
+    mVsyncBridge = nullptr;
+  }
 }
 
 RefPtr<CompositorSession>

gfx/ipc/VsyncBridgeChild.cpp

@@ -114,8 +114,14 @@ VsyncBridgeChild::Close()
   if (!mProcessToken) {
     return;
   }
-  PVsyncBridgeChild::Close();
+
+  // Clear the process token so we don't notify the GPUProcessManager. It already
+  // knows we're closed since it manually called Close, and in fact the GPM could
+  // have already been destroyed during shutdown.
   mProcessToken = 0;
+
+  // Close the underlying IPC channel.
+  PVsyncBridgeChild::Close();
 }
 
 void