Fix defective sync token logic in OffscreenCanvasResourceProvider

When resource type is an AcceleratedStaticBitmapImage, the
WaitSyncTokenCHROMIUM call in the release callback is skipped because the
context_provider_wrapper_ is null. This change fixes the problem by setting
the context_provider_wrapper_ field of the frame resource to the same context
that own the image resource.

BUG=769161

Change-Id: Ib05a0fa5a6a0d4dab676fcc27820d1171260a25d
Reviewed-on: https://chromium-review.googlesource.com/685380Reviewed-by: Olivia Lai <xlai@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505388}

third_party/WebKit/Source/platform/graphics/OffscreenCanvasResourceProvider.cpp

@@ -165,6 +165,7 @@ void OffscreenCanvasResourceProvider::
 
   std::unique_ptr<FrameResource> frame_resource =
       CreateOrRecycleFrameResource();
+  frame_resource->context_provider_wrapper_ = image->ContextProviderWrapper();
   frame_resource->image_ = std::move(image);
   resources_.insert(next_resource_id_, std::move(frame_resource));
 }