Disallow script execution in Document::detach().

There's no reason to run scripts here, since unload events have already
run. Allowing scripts here can lead to re-entrant frame detach, which
results in lots of weird edge cases that Blink wouldn't have to handle
otherwise.

BUG=363099,371084

Review URL: https://codereview.chromium.org/657263002

git-svn-id: svn://svn.chromium.org/blink/trunk@184035 bbb929c8-8fbe-4397-9dbb-9b2b20218538