Bug 1199885 - Part 5: Make mRootLayerTreeID const to prove that there's no data races. r=kats

--HG--
extra : commitid : 5WndwzOuLdi

gfx/layers/ipc/CompositorParent.cpp

@@ -548,6 +548,7 @@ CompositorParent::CompositorParent(nsIWidget* aWidget,
   , mEGLSurfaceSize(aSurfaceWidth, aSurfaceHeight)
   , mPauseCompositionMonitor("PauseCompositionMonitor")
   , mResumeCompositionMonitor("ResumeCompositionMonitor")
+  , mRootLayerTreeID(AllocateLayerTreeId())
   , mOverrideComposeReadiness(false)
   , mForceCompositionTask(nullptr)
   , mCompositorThreadHolder(sCompositorThreadHolder)
@@ -573,7 +574,6 @@ CompositorParent::CompositorParent(nsIWidget* aWidget,
 
   CompositorLoop()->PostTask(FROM_HERE, NewRunnableFunction(SetThreadPriority));
 
-  mRootLayerTreeID = AllocateLayerTreeId();
 
   { // scope lock
     MonitorAutoLock lock(*sIndirectLayerTreesLock);

gfx/layers/ipc/CompositorParent.h

@@ -474,7 +474,7 @@ protected:
   mozilla::Monitor mResumeCompositionMonitor;
 
   uint64_t mCompositorID;
-  uint64_t mRootLayerTreeID;
+  const uint64_t mRootLayerTreeID;
 
   bool mOverrideComposeReadiness;
   CancelableTask* mForceCompositionTask;