device/bluetooth: Move GATT observer methods to BluetoothAdapter::Observer.

device/bluetooth: Move GATT observer methods to BluetoothAdapter::Observer.

This CL merges BluetoothDevice::Observer and BluetoothGattService::Observer
interfaces to BluetoothAdapter::Observer. This makes adding and removing
observers much cleaner for users of the library. It also removes the problem of
adding observers on platforms where instances are temporary and aren't kept
alive for long.

BUG=384664
TEST=device_unittests; browser_tests:BluetoothLowEnergyApiTest

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=286371

[armansito, 2014-07-23 18:24:06]

rpaquay@: cc'd FYI

[keybuk, 2014-07-29 17:31:53]

lgtm

this looks much cleaner!

[armansito, 2014-07-29 19:10:39]

The CQ bit was checked by armansito@chromium.org

[commit-bot: I haz the power, 2014-07-29 19:12:12]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/armansito@chromium.org/418483003/1

[commit-bot: I haz the power, 2014-07-30 01:21:42]

Change committed as 286371