device/bluetooth: Add abstract API definition for BluetoothGattService.

This CL defines the abstract class BluetoothGattService, which represents
remote and local GATT services in the GATT API.

BUG=340525
TEST=Compile Chrome.

Review URL: https://codereview.chromium.org/148203013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249914 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249914