[Extensions] Fix keyboard focus behavior for activity log page

This CL fixes keyboard focus for the activity log page by introducing
these 2 behaviors:

1) When the activity log page is entered either through the detail view
or via url, the back button is focused.

2) When the user navigates back from the activity log page to the detail
view page, the View Activity Log button/link is focused.

Patchset >1 achieves this by using a flag stored in manager.js which
indicates if the previous view was the activity log. This flag is set to
true by an event which is fired when the user is about to exit the
activity log and is set back to false by manager.js in onViewEnterStart.

The detail view page uses the flag upon onViewEnterStart to determine
which element should receive focus. This is safe as onViewEnterStart
is bubbled up in manager.js later so the flag is safely reset.

Bug: 908598
Change-Id: I58bfda628aa347357315f483f3859856c7264196
Reviewed-on: https://chromium-review.googlesource.com/c/1410123
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623551}
9 files changed