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In the ozone drm platform, the ChannelObserver watches for changes
on the IPC channel. However, ozone/drm will soon support execute in
an in-process mode for MUS and the existing multi-process mode.
Rename the class to reflect this.

BUG=558602

Review URL: https://codereview.chromium.org/1603213002

Cr-Commit-Position: refs/heads/master@{#371920}