Battery Status API: add testing support for LayoutTests.

Adds plumbing to make it possible to test the blink
implementation of the Battery Status API using LayoutTests and
TestRunner. In test mode a fake battery status dispatcher is
used to allow manual triggering of battery status changes using
TestRunner.didChangeBatteryStatus method.

BUG=122593, 360068

Review URL: https://codereview.chromium.org/298263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274652 0039d316-1c4b-4281-b951-d872f2087c98
12 files changed