Disallowing sync XHR in pagedimissal.

Disallow sync XHR during page dismissal when the page is being navigated
away or closed by the user. Sync XHR is already on the deprecation path and
its usage hurts end user experience.

Rolling this out behind a base::Feature to help mitigate risk. This feature
is disabled by default.

Bug: 827324
Change-Id: I32cb71c3896281c1311ee32de3e7e434ba80322b
Reviewed-on: https://chromium-review.googlesource.com/c/1336914
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619511}
10 files changed