Prevent frame from being destroyed in middle of notification.

Prevent frame from being destroyed in middle of notification.

Extension injected scripts might trigger frame deletion during handling of its dispatchDidFinishDocumentLoad() function.

BUG=328342

[dcheng, 2014-02-14 20:38:30]

+eseidel for the actual review. Here are some of my thoughts:

This is kind of unfortunate for several reasons:
1) Not only does WebCore have to self-protect this (WebCore::Frame), the
embedder sometimes needs to protect this as well (blink::WebFrameImpl). Yuck!
Hopefully this doesn't need to bubble back into content::RenderFrameImpl as
well... but even with this patch, a corresponding patch in content is needed to
null check a bunch of random things that might be cleared if WebFrameImpl is
destroyed (see https://codereview.chromium.org/112203003/).

2) The behavior might be different from before.
Before: I think all user scripts would finish executing, even if one of them
removes the frame.
After: If one of the user scripts removes the frame, all the user scripts that
happen to execute after it will do nothing.

3) If we can allocate WebFrameImpl on the Oilpan heap, maybe this will be less
painful? But then we need to make sure the content references to WebFrameImpl is
visible to Oilpan's tracer...

[mharanczyk, 2014-02-25 10:36:43]

Added similar protector to documentElementAvailable().