GitLab

Move the WebContentsObserver from TabLifeCycleUnit to TabLifeCycleUnitSource,
this allows for a better tracking of the WebContents lifetime, in some
situation a WebContents might get detached from the TabStrip and then
destroyed, which mean that we won't get a TabClosingAt notification for
this tab destruction.

Another solution would be to implement the TabStripModelObserver::TabDetachedAt
function and track the tabs which are in a detached state but this is slightly
more complex because TabDetachedAt might be called for several reasons:
- A TabDetachedAt usually come after a TabClosedAt event.
- TabDetachedAt might be followed by TabInsertedAt, or not if it get destroyed.
  because of this we won't know if we should keep the TabLifeCycleUnit for this
  WebContents around (i.e. if it'll get re-inserted in a tab strip) or destroy
  it because it's being destroyed.

Observing WebContentsObserver::WebContentsDestroyed and moving the logic that
was in TabClosingAt to this method address these issues, it's the same approach
than the one we took in TabStatsTracker.

Bug: 819352, 818454
Change-Id: Ibd3fe49b2798ade19ee781cb70eb30e52372d686
Reviewed-on: https://chromium-review.googlesource.com/952405
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541540}