mus: Remove aura version of WindowCompositorFrameSink.

The two versions of WindowCompositorFrameSink, in aura-mus, and in the
old client-lib, are identical. Remove the version in aura-mus, and retain
the one from the old client-lib. This allows its use from chrome-renderer
without pulling in the rest of aura.

BUG=672913

Review-Url: https://codereview.chromium.org/2647363002
Cr-Commit-Position: refs/heads/master@{#445472}
10 files changed