GitLab

Make session observer to be ScopedObserver so it remove itself when out
of scope.
Also add AllowJavascript in some JS call hander.

Bug: 876467
Change-Id: If7cd1c2fe42024708027bee5f87855244c7a4ea9
Reviewed-on: https://chromium-review.googlesource.com/1183590Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Xiaoyin Hu <xiaoyinh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585094}