Use WeakPtr for MarkProxiesAsBad callback

This was causing a crash if the throttle was destroyed before the
callback was called.

Bug: 914436
Change-Id: Id52e41c2f42fe29116a2b877311cc1d9405b5116
Reviewed-on: https://chromium-review.googlesource.com/c/1374159
Reviewed-by: rajendrant <rajendrant@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616003}
2 files changed