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and OnNavigationCompleted.

Content does not allow WebContents to be deleted from
WebContentsObserver::DidFinishNavigation(). Unfortunately we
keep seeing crashes because of this restriction, so I'm
inclined to provide some mitigation.

This patch takes the approach of detecting this scenario
and delaying deletion of the WebContents. All the surrounding
WebLayer classes are deleted/destroyed and ownership of the
WebContents is passed to the Profile and deleted from a
post-task.

Doing this may introduce it's own set of problems. For example,
if a callback/notification is still in flight from content that gets
processed before the deletion, and the code assumes there is a Tab
associated with the WebContents, then there will be problems.

My preference is still to fix this in content, but in the mean
time this should work.

BUG=1111127
TEST=covered by test

Change-Id: I9dbd9f90b88cbdbdfbab9aab81840b986fcdc410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377552
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802416}