Mac: fix sensor data fetcher bug when accelerometer sensor is not available.

Return false from the Start() method when the SuddenMotionSensor
initialization fails on the Mac plaform.

BUG=297451
NOTRY=true
TBR=mvanouwerkerk@chromium.org, bulach@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24360018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225000 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225000