Protect FrameView in Document::updateLayout().

It's possible to dispatch events in postAttachCallbacks, and an event handler
could delete the FrameView.

BUG=324321
TEST=none; could not make a reliable test.

Review URL: https://codereview.chromium.org/98513002

git-svn-id: svn://svn.chromium.org/blink/trunk@162964 bbb929c8-8fbe-4397-9dbb-9b2b20218538