Enable Device Motion API in Blink by default.

We are now ready to enable Device Motion API for
all platforms. This patch sets DeviceMotion status
to stable.

For the corresponding patch to enable Device Motion
API on the Chromium side see:
https://codereview.chromium.org/24220002/

BUG=135804

Review URL: https://chromiumcodereview.appspot.com/23578042

git-svn-id: svn://svn.chromium.org/blink/trunk@158031 bbb929c8-8fbe-4397-9dbb-9b2b20218538