Fix heap-use-after-free by using weak factory instead of Unretained

Bug: 856578
Change-Id: Ifb2a1b7e6c22e1af36e12eedba72427f51d925b9
Reviewed-on: https://chromium-review.googlesource.com/1114617
Reviewed-by: Hector Dearman <hjd@chromium.org>
Commit-Queue: Hector Dearman <hjd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571528}