GitLab

When opening a new tab for the same origin as a the current tab,
the new tab was incorrectly linked to the currently open tab's
MediaEngagementSession.

This adds a check to only allow the sessions to be linked if the
new tab navigation originated from a link on the current tab.

BUG=819807

Change-Id: I02f892372f1aa92f830f891a369732a4fc7b8883
Reviewed-on: https://chromium-review.googlesource.com/962242
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544026}