Reland "Pausable Object Cleanup - XMLHttpRequest"

This is a reland of df0477e9ca0423600017a6e6fb566ff5784c7667

Now that Workers pause correctly this change can be relanded.

Original change's description:
> Pausable Object Cleanup - XMLHttpRequest
>
> The events dispatched by the XMLHttpRequestEventThrottle are attached
> to the kNetwork TaskQueue which is a pausable task queue.
>
> Verified progress events aren't received while an alert dialog is present.
>
> BUG=907125
>
> Change-Id: I93d8bbf8151753c5ddfc14bbfdd067697f6fe01e
> Reviewed-on: https://chromium-review.googlesource.com/c/1368462
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Alexander Timin <altimin@chromium.org>
> Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#615143}

TBR=haraken@chromium.org,altimin@chromium.org

Bug: 907125
Change-Id: Ie6b7f8be11eb436a23456dc563b6dfacf26ac703
Reviewed-on: https://chromium-review.googlesource.com/c/1384652
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617875}
4 files changed