Fix slow memory leak in CanvasSurfaceLayerBridge

Graphics layers were not getting unregistered so the layer id of
the CanvasSurfaceLayerBridge's web_layer_ was getting leaked.

BUG=735029

Change-Id: If8ab2badba2fc25243834939e9c95d3c9582c3c0
Reviewed-on: https://chromium-review.googlesource.com/541496Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480872}

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

@@ -83,6 +83,9 @@ CanvasSurfaceLayerBridge::CanvasSurfaceLayerBridge(
 
 CanvasSurfaceLayerBridge::~CanvasSurfaceLayerBridge() {
   observer_ = nullptr;
+  if (web_layer_) {
+    GraphicsLayer::UnregisterContentsLayer(web_layer_.get());
+  }
 }
 
 void CanvasSurfaceLayerBridge::CreateSolidColorLayer() {