Implement PollingThread for Device Motion.

On some platforms data fetchers are poll-based (e.g. mac), so it requires a polling thread to fetch data.
This CL includes:
- implementation of a provider for device motion with a polling thread.
- unit test for provider
- make sure to call Stop* in fetcher destructors only when started.

BUG=135804

Review URL: https://chromiumcodereview.appspot.com/19833005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@214538 0039d316-1c4b-4281-b951-d872f2087c98
7 files changed