Update deviceorientation's notes about chrome 31

Chromium/Blink supports the devicemotion event. It's available in Chromium 31.0.1639.0 (224567).
https://src.chromium.org/viewvc/blink?revision=158031&view=revision
https://src.chromium.org/viewvc/chrome?revision=224164&view=revision
Merge pull request #306 from syoichi/pr9-deviceorientation

Update deviceorientation's notes about chrome 31