Disallow scripting earlier in LocalFrame::detach().

      The Chrome extension implementation no longer needs to run script in
WebFrameClient::willReleaseScriptContext(), so forbid scripting earlier
in detach to make it harder to break invariants.

BUG=534923,555773

Review URL: https://codereview.chromium.org/1657583002

Cr-Commit-Position: refs/heads/master@{#372611}