Check new display item list committed when destructing PaintController

BUG=616700

Review-Url: https://codereview.chromium.org/2038873002
Cr-Commit-Position: refs/heads/master@{#398236}

third_party/WebKit/Source/platform/graphics/paint/PaintController.h

@@ -44,6 +44,8 @@ public:
 
     ~PaintController()
     {
+        // New display items should be committed before PaintController is destructed.
+        DCHECK(m_newDisplayItemList.isEmpty());
 #if CHECK_DISPLAY_ITEM_CLIENT_ALIVENESS
         DisplayItemClient::endShouldKeepAliveAllClients(this);
 #endif