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/2026823003 .

Cr-Commit-Position: refs/branch-heads/2743@{#153}
Cr-Branched-From: 2b3ae3b8090361f8af5a611712fc1a5ab2de53cb-refs/heads/master@{#394939}
4 files changed