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This is needed as we create platform surfaces on gpu main and therefore
must be able to respond to messages from the winsys on that thread.

Bug: 1047032

Change-Id: Ice667cb451ae17a9e10fbc157f0b88aa0bf14c65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2047494
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740097}