mus: Turn off using BeginFrameSource in ui::Compositor.
mus: Turn off using BeginFrameSource in ui::Compositor.

When running as a mus-client, disable using BeginFrameSource, as
mus doesn't tell its clients about BeginFrame yet.

BUG=603600

Review URL: https://codereview.chromium.org/1888093002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#387417}

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