Move the NULLing out of WebViewImpl::delegate_ until after we close the page.

Move the NULLing out of WebViewImpl::delegate_ until after we close the page.
This allows us to receive notifications like when the V8 context is destroyed.

BUG=12686
TEST=no

[Matt Perry, 2009-07-01 22:29:35]

There aren't many options as to where to move the NULLing of delegate_.  I could
do it at the end of WebFrameLoaderClient::detachedFromParent3, which is just
after the context-destroyed event is sent.  A small amount of page cleanup
happens after that, and then code execution returns to WebViewImpl::Close().

The other option is to not NULL it out at all, but this seemed slightly safer,
since some delegate callbacks occur in ~WebViewImpl, when objects are being
destroyed.

I ran the layout tests locally and there were no unexpected failures.

[darin (slow to review), 2009-07-02 03:22:32]

LGTM