Bug 801176 - part8-v1: Fix memory leak in OffscreenCanvas. r=roc

---
 dom/canvas/CanvasRenderingContext2D.cpp |  2 ++
 dom/canvas/OffscreenCanvas.cpp          | 12 +++++++-----
 2 files changed, 9 insertions(+), 5 deletions(-)

dom/canvas/CanvasRenderingContext2D.cpp

@@ -886,6 +886,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(CanvasRenderingContext2D)
   // since we're logically destructed at this point.
   CanvasRenderingContext2D::RemoveDemotableContext(tmp);
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mCanvasElement)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mOffscreenCanvas)
   for (uint32_t i = 0; i < tmp->mStyleStack.Length(); i++) {
     ImplCycleCollectionUnlink(tmp->mStyleStack[i].patternStyles[Style::STROKE]);
     ImplCycleCollectionUnlink(tmp->mStyleStack[i].patternStyles[Style::FILL]);
@@ -903,6 +904,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(CanvasRenderingContext2D)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mCanvasElement)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOffscreenCanvas)
   for (uint32_t i = 0; i < tmp->mStyleStack.Length(); i++) {
     ImplCycleCollectionTraverse(cb, tmp->mStyleStack[i].patternStyles[Style::STROKE], "Stroke CanvasPattern");
     ImplCycleCollectionTraverse(cb, tmp->mStyleStack[i].patternStyles[Style::FILL], "Fill CanvasPattern");

dom/canvas/OffscreenCanvas.cpp

@@ -89,11 +89,13 @@ OffscreenCanvas::ClearResources()
     mCanvasClient = nullptr;
 
     if (mCanvasRenderer) {
-      nsCOMPtr<nsIThread> activeThread = mCanvasRenderer->GetActiveThread();
-      MOZ_RELEASE_ASSERT(activeThread);
-      bool current;
-      activeThread->IsOnCurrentThread(&current);
-      MOZ_RELEASE_ASSERT(current);
+      if (mCanvasRenderer->mGLContext) {
+        nsCOMPtr<nsIThread> activeThread = mCanvasRenderer->GetActiveThread();
+        MOZ_RELEASE_ASSERT(activeThread);
+        bool current;
+        activeThread->IsOnCurrentThread(&current);
+        MOZ_RELEASE_ASSERT(current);
+      }
       mCanvasRenderer->SetCanvasClient(nullptr);
       mCanvasRenderer->mContext = nullptr;
       mCanvasRenderer->mGLContext = nullptr;