device/bluetooth: Improvements to GATT descriptor access API.

device/bluetooth: Improvements to GATT descriptor access API.

This CL introduces the following changes to the GATT API:

* New BluetoothGattService::Observer methods:
 - GattDescriptorAdded
 - GattDescriptorRemoved
 - GattDescriptorValueChanged

* New BluetoothGattCharacteristic::GetDescriptor method to obtain a
characteristic's descriptor by descriptor identifier.

BUG=265663
TEST=device_unittests

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269138

[keybuk, 2014-05-06 16:45:55]

lgtm

[armansito, 2014-05-07 19:00:21]

The CQ bit was checked by armansito@chromium.org

[commit-bot: I haz the power, 2014-05-07 19:03:30]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/armansito@chromium.org/264053004/20001

[commit-bot: I haz the power, 2014-05-08 05:53:23]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  win_chromium_rel on tryserver.chromium

[commit-bot: I haz the power, 2014-05-08 18:53:19]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-08 18:53:19]

Try jobs failed on following builders:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[armansito, 2014-05-08 19:31:26]

The CQ bit was checked by armansito@chromium.org

[commit-bot: I haz the power, 2014-05-08 19:34:17]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/armansito@chromium.org/264053004/20001

[commit-bot: I haz the power, 2014-05-09 01:42:27]

Change committed as 269138