SafeBrowsing URL checks: supporting notification of starting slow checks.

This notification will be used by the URL check initiator (e.g.,
ThrottlingURLLoader) to ask the network service to pause reading response body.
It is useful to reduce the time window during which the network service could
write potentially unsafe resources into disk cache.

Bug: 715673
Change-Id: Ibc606998a58709df8901a85915942af0d8986f83
Reviewed-on: https://chromium-review.googlesource.com/661888
Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502107}
11 files changed