Disable frame navigations during DocumentLoader detach in FrameLoader::startLoad

BUG=613266

Review-Url: https://codereview.chromium.org/2006033002
Cr-Commit-Position: refs/heads/master@{#396241}
(cherry picked from commit 1948aefa8901dca0ccb993753fca00b15d2a6e25)

Review URL: https://codereview.chromium.org/2021373003 .

Cr-Commit-Position: refs/branch-heads/2704@{#687}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
4 files changed