Disable sub frame loads more reliably in frame detach.

Previously, guarding against attaching a new frame in frame detach
was done by inspecting LoadEventProgress. There are certain
operations, such as document.open(), that can cause LoadEventProgress
to never advance, causing the guard to get skipped.


Review URL: https://codereview.chromium.org/1659013003

Cr-Commit-Position: refs/heads/master@{#373581}
6 files changed