GitLab

If the MediaStreamManager were destroyed after a MediaStreamUi was
created but before it started recording, it wouldn't call OnManagerGone
on the MediaStreamUi.

Switch to using a WeakPtr.

Bug: 1147646
Change-Id: I72010ed73daa26c4f682e2779efe046d1ddb2020
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532966Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826454}