Disable navigations in the unload handler.

Blink intent:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/VfItzNe3WO0

BUG=613244

Review-Url: https://codereview.chromium.org/2206843003
Cr-Commit-Position: refs/heads/master@{#414588}
7 files changed