Fix issue where iframe content wasn't included in the accessibility tree.

Source/core/rendering/RenderWidget.cpp

Index: Source/core/rendering/RenderWidget.cpp
diff --git a/Source/core/rendering/RenderWidget.cpp b/Source/core/rendering/RenderWidget.cpp
index 936d7db40d561be88fa6020085f5260c10f98d14..3e4aa0a20dab50c9993c91b941e3c3bca78417b9 100644
--- a/Source/core/rendering/RenderWidget.cpp
+++ b/Source/core/rendering/RenderWidget.cpp
@@ -204,6 +204,9 @@ void RenderWidget::setWidget(PassRefPtr<Widget> widget)
         }
         moveWidgetToParentSoon(m_widget.get(), m_frameView);
     }
+
+    if (AXObjectCache* cache = document()->existingAXObjectCache())

[aboxhall, 2013/05/22 16:05:10]

Can you help me understand under what circumstances the cache would or would not
exist? Or is this just a defensive check?

[dmazzoni, 2013/05/22 16:28:13]

The cache shouldn't exist if accessibility is disabled in this
renderer, or if this document is not yet attached to the page.

We could check AXObjectCache::accessibilityEnabled() - however
if we did that we'd probably still want to call
existingAXObjectCache and check if it exists, because there
are some corner cases where accessibility is enabled but we
don't want an AXObjectCache for one particular document (like
if that document is not yet attached to the page).

So most of the time, code that needs to update the accessibility
tree can just do so if there's an AXObjectCache for the current
document and not otherwise. In just a few places we carefully
create the cache if it doesn't already exist.

+        cache->childrenChanged(this);
 }
 
 void RenderWidget::layout()