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For Ozone, there is no default platform. Since the unittests use their own
delegates this simplifies things by not requiring an Ozone platform when
executing the unittests.

BUG=none

Review URL: https://codereview.chromium.org/383463002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282188 0039d316-1c4b-4281-b951-d872f2087c98