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}
4 files changed