Mark entries to be skipped on back for history manipulation intervention.

Entries that are added to the back/forward list without the user's intention
are marked to be skipped on subsequent back button invocations.
This CL only adds the bit and subsequent CLs will add metrics and the intervention
logic based on this bit.

Test: content_browsertests --gtest_filter=*SkipOnBack*
Bug: 907167
Change-Id: I169031de2c977b53605960b8baf0cd1f6ac4ef47
Reviewed-on: https://chromium-review.googlesource.com/c/1344199
Commit-Queue: Shivani Sharma <shivanisha@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615904}
8 files changed