Add Battery Status API support in content/renderer and IPC messages.

This patch defines the async IPCs needed for the Battery Status API.
It also implements the platform-side part with a
setBatteryStatusListener() method and a dispatcher class for
propagating messages to blink. Unit tests for the dispatcher
class are also included.

BUG=122593,360068

Review URL: https://codereview.chromium.org/252113006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270504 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed