Lose/restore WebGL contexts if multisampling blackist status changes at runtime.

Source/core/page/Page.cpp

Index: Source/core/page/Page.cpp
diff --git a/Source/core/page/Page.cpp b/Source/core/page/Page.cpp
index 8296aa459215a3794cdff5acb962abd45b108b18..3dce2d14aceaa2f6e6ac173d3da4e09bebb45631 100644
--- a/Source/core/page/Page.cpp
+++ b/Source/core/page/Page.cpp
@@ -826,6 +826,24 @@ void Page::captionPreferencesChanged()
         frame->document()->captionPreferencesChanged();
 }
 
+Page::MultisamplingChangedObserver::MultisamplingChangedObserver(Page* page)
+    : m_page(page)
+{
+    m_page->m_multisamplingChangedObservers.add(this);
+}
+
+Page::MultisamplingChangedObserver::~MultisamplingChangedObserver()
+{
+    m_page->m_multisamplingChangedObservers.remove(this);
+}

[Ken Russell (switch to Gerrit), 2013/05/08 02:17:02]

It's confusing to me that the base class MultisamplingChangedObserver adds and
removes itself from the Page's hash set. I would recommend instead making
MultisamplingChangedObserver a pure virtual class with a single method, add
add/removeMultisamplingChangedObserver methods to Page, and just make
WebGLRenderingContext itself implement MultisamplingChangedObserver, and remove
itself from the Page's set in its destructor (since Page outlives
WebGLRenderingContext).

+
+void Page::multisamplingChanged()
+{
+    HashSet<MultisamplingChangedObserver*>::iterator stop = m_multisamplingChangedObservers.end();
+    for (HashSet<MultisamplingChangedObserver*>::iterator it = m_multisamplingChangedObservers.begin(); it != stop; ++it)
+        (*it)->multisamplingChanged(m_settings->openGLMultisamplingEnabled());

[abarth-chromium, 2013/05/07 22:10:59]

Can this call end up executing JavaScript or destroying the Page?

[Ken Russell (switch to Gerrit), 2013/05/08 02:17:02]

Actually, it's guaranteed that it doesn't. It starts a timer which dispatches an
event to JavaScript.

+}
+
 Page::PageClients::PageClients()
     : alternativeTextClient(0)
     , chromeClient(0)