[Chromecast] Replace Unretained(this) with WeakPtr in MediaPipelineBackendForMixer.

BUG=internal b/117194691

Test=Cast from Youtube to Google Home Hub.(Since we could not reproduce
the bug in the office, this is only a general cast test.)

Change-Id: I2f9ac0079bc3d5846dcd5f67a860796b7340fb6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529998
Commit-Queue: Ziyang Cheng <ziyangch@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642660}
2 files changed