Add checks to try to catch unexpected re-entrancy during frame detach.

BUG=363099

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173187

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/269673004

git-svn-id: svn://svn.chromium.org/blink/trunk@173938 bbb929c8-8fbe-4397-9dbb-9b2b20218538