Change GPU process to use IO rather than UI MessageLoop on Mac OS. On this platform, the GPU process does not need to access any on-screen resources.
Change GPU process to use IO rather than UI MessageLoop on Mac OS. On this platform, the GPU process does not need to access any on-screen resources.

This is an attempted workaround for
http://code.google.com/p/chromium/issues/detail?id=131310 .

BUG=131310
TEST=ran WebGL content, Poster Circle, and Flash content on Mac OS X


Review URL: https://chromiumcodereview.appspot.com/10581041

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@143263 0039d316-1c4b-4281-b951-d872f2087c98

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