GitLab

The extension TabHelper can have an associated extension if the
WebContents holds a hosted app. Watch for that extension being
unloaded and reset the reference to the extension if it is so that
we don't UAF it.

BUG=616057
BUG=616113

Review-Url: https://codereview.chromium.org/2037873002
Cr-Commit-Position: refs/heads/master@{#398072}