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This patch is to fix this scenario:
* Device: DidModifyServices notification
    - Chrome: Scan for primary services
* Device: DidDiscoverServices
    - Chrome: Scan for characteristics
* Device: DidModifyServices notification
    - Chrome: Scan for primary services
* Device: DidDiscoverCharacteristics
* Device: DidDiscoverServices
    - Chrome: Scan for characteristics
* Device: DidDiscoverCharacteristics

We need to wait until all the pending characteristic discoveries are done until we can start scanning
for descriptors. We need to make sure descriptors are not ready if DidModifyServices notification
is received while scanning for descriptors.

Adding discovery_pending_count_ in BluetoothRemoteGattServiceMac and BluetoothRemoteGattCharacteristicMac.
|is_discovery_complete_| can be set to true when discovery_pending_count_ is equal to 0.

BUG=690204

Review-Url: https://codereview.chromium.org/2638653002
Cr-Commit-Position: refs/heads/master@{#469457}