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Add a method to ContentBrowserClient. Note this controls gpu process
"launch" for in-process GPU as well. The method still allows using an
existing GPU process, so in-process case should be minimally impacted so
probably not worth carving out special conditions for it.

For now, only call this new API from BrowserGpuChannelHostFactory, which
controls GPU clients in the browser process. Clients in child process is
more risky, so holding going to do that part in a follow up CL instead.

ChromeContentBrowserClient on android implements the API by checking if
there are running or paused activities. Paused activity may still be
visible, so allow it to launch GPU in that case.

Bug: 779211
Change-Id: Ic48acc9f9ab6134371d7f193d9031e912a05d947
Reviewed-on: https://chromium-review.googlesource.com/783495Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519529}