Convert MediaWebContentsObsever to be the client of WakeLock mojo interface.

Convert MediaWebContentsObsever to be the client of WakeLock mojo interface
instead of direct client of PowerSaveBlocker.

TEST = Play audio or video and check the OS level wake locks.
On macOS: pmset -g assertions
On Windows: powercfg /requests.

BUG=689413
TBR=<jam@chromium.org>

Review-Url: https://codereview.chromium.org/2846813002
Cr-Commit-Position: refs/heads/master@{#470889}
3 files changed