Blink-side Device Motion API testing.

In this testing approach we inject a mock device motion data object
into platform and catch the 'ondevicemotion' event in Javascript, 
with that same data.

BUG=135804

Review URL: https://chromiumcodereview.appspot.com/16020003

git-svn-id: svn://svn.chromium.org/blink/trunk@154502 bbb929c8-8fbe-4397-9dbb-9b2b20218538