[Bluetooth][WinRT] Implement Obtaining Gatt Descriptors

This change implements obtainining Remote Gatt Descriptors for WinRT by
hooking up the appropriate logic into GattDiscovererWinrt. Furthermore,
appropriate tests are enabled.

Bug: 821766
Change-Id: I20475478367c2ab6b18940b4271de70ec138f676
Reviewed-on: https://chromium-review.googlesource.com/1156392
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579830}
22 files changed