[Chromium] debug builds: Use after free under ~PageOverlay()

Downstream bug: http://crbug.com/157097

What steps will reproduce the problem?
0. Run a debug build of Chromium
1. Open a packaged app Shell window, or popout a Google Chat Panel window
2. Right-click -> Inspect Element, Developer tools opens in new window
3. Close the original Shell/Panel Window
4. segfault around:

 void GraphicsLayer::willBeDestroyed()
 {
 #ifndef NDEBUG
     if (m_client)
         m_client->verifyNotPainting();
 #endif


This code added in bug 81954 (from March)


The problem is that WebKit::PageOverlay is declared like this:

class PageOverlay {
/* snip */

    WebViewImpl* m_viewImpl;
    WebPageOverlay* m_overlay;
    OwnPtr<WebCore::GraphicsLayer> m_layer;
    OwnPtr<WebCore::GraphicsLayerClient> m_layerClient;
    int m_zOrder;
};

So m_layerClient will be destroyed before m_layer at the end of ~PageOverlay().

So, it is not valid for GraphicsLayer::willBeDestroyed (called from the ~GraphicsLayerChromium leaf class) to access its own GraphicsLayerClient data member in this situation to do m_client->verifyNotPainting();.

Some possible solutions:
 - Fix the destruction order (reorder members)
 - explicitly set PageOverlay::m_layer->m_client to NULL in ~PageOverlay
 - something-else-because-i-don't-really-know-webkit-that-well

Created an attachment (id=171850) [details]
Patch

Created an attachment (id=171851) [details]
Patch

(From update of attachment 171851 [details])
Clearing flags on attachment: 171851

Committed r133188: <http://trac.webkit.org/changeset/133188>

All reviewed patches have been landed.  Closing bug.