Check toWebPluginContainerImpl() result before dereferencing, even with...

Check toWebPluginContainerImpl() result before dereferencing, even with isPluginDocument() sometimes it's 0;

BUG=331927

Review URL: https://codereview.chromium.org/207143003

git-svn-id: svn://svn.chromium.org/blink/trunk@169825 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/web/WebPluginDocument.cpp

@@ -49,7 +49,7 @@ WebPlugin* WebPluginDocument::plugin()
         return 0;
     PluginDocument* doc = unwrap<PluginDocument>();
     WebPluginContainerImpl* container = toWebPluginContainerImpl(doc->pluginWidget());
-    return container->plugin();
+    return container ? container->plugin() : 0;
 }