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Uses the MessagePumpFactory functionality to install a default message pump in
unit tests.  This allows us to stop hard-coding a default message pump in many
separate tests and instead centralizes the logic.  When tests ask for a
MessageLoopForUI now, they will actually receive a message loop that is backed
by a default message pump, which means that calling Run() and Quit() will work
without crashing.

BUG=None
TEST=None

Review URL: https://chromiumcodereview.appspot.com/10701172

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148384 0039d316-1c4b-4281-b951-d872f2087c98