GitLab

This avoid casting CmaBackend insteaces in MediaPipelineBackendManager,
which is unsafe since we don't know the real type.

Change-Id: I82ae529384575f2db5002ab1417de4a056eb3186
Reviewed-on: https://chromium-review.googlesource.com/1159221Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579938}