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When the GPU process dies/gets killed, the Send() can fail (e.g. EPIPE),
triggering lost context recreation, before OnChannelError gets called (happening
when we return to the message loop) which ends up deleting the GpuProcessHost.
So, in the mean time, make sure we don't try to reuse that GpuProcessHost to
recreate a channel.

BUG=129067


Review URL: https://chromiumcodereview.appspot.com/10834165

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149984 0039d316-1c4b-4281-b951-d872f2087c98