New test fixture for DeviceMotionEventPump to avoid PlatformThread::Sleep

Implement a test fixture in DeviceMotionEventPumpTest to consistently
test DeviceMotionEventPump. The new implementation also avoids calls
to PlatformThread::Sleep() since it is generally undesirable in tests.

BUG=333928

Review URL: https://codereview.chromium.org/134073002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245286 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245286