Add page visibility observer support for Device Motion

Stop firing 'ondevicemotion' events when a tab goes invisible to optimize
battery life.

R=timvolodine@chromium.org
BUG=263411
TEST=fast/dom/DeviceMotion/page-visibility.html

Review URL: https://chromiumcodereview.appspot.com/23705010

git-svn-id: svn://svn.chromium.org/blink/trunk@157968 bbb929c8-8fbe-4397-9dbb-9b2b20218538