Oilpan: hide persisted plugin object before disposing.

When clearing out the persisted plugin object, hide the object before
going ahead with the disposal. This mirrors what will happen non-Oilpan.

R=haraken
BUG=561473

Review URL: https://codereview.chromium.org/1475023002

Cr-Commit-Position: refs/heads/master@{#361665}

third_party/WebKit/Source/core/html/HTMLPlugInElement.cpp

@@ -114,8 +114,10 @@ void HTMLPlugInElement::setPersistedPluginWidget(Widget* widget)
 #if ENABLE(OILPAN)
     unregisterAsRenderlessIfNeeded();
     registerAsRenderless(widget);
-    if (m_persistedPluginWidget)
+    if (m_persistedPluginWidget) {
+        m_persistedPluginWidget->hide();
         m_persistedPluginWidget->dispose();
+    }
 #endif
     m_persistedPluginWidget = widget;
 }