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We can't call set_client on the main thread because that determines the thread
affinity of the SurfacePtr. Instead, we need to wait for the BindToClient
callback so we can bind the SurfacePtr on the impl thread.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/603663002

Cr-Commit-Position: refs/heads/master@{#296474}