Bluetooth: macOS: DidModifyServices can happens while scanning

device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm

Index: device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm
diff --git a/device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm b/device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm
index 8e4a14dd52d75dfdf3866ffe1f357e58f7fb30ca..61432ab9d8b3e41bb97d3526fd8419043fe3258c 100644
--- a/device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm
+++ b/device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm
@@ -69,7 +69,9 @@ static BluetoothGattCharacteristic::Properties ConvertProperties(
 BluetoothRemoteGattCharacteristicMac::BluetoothRemoteGattCharacteristicMac(
     BluetoothRemoteGattServiceMac* gatt_service,
     CBCharacteristic* cb_characteristic)
-    : gatt_service_(gatt_service),
+    : is_discovery_complete_(false),
+      discovery_pending_count_(0),
+      gatt_service_(gatt_service),
       cb_characteristic_(cb_characteristic, base::scoped_policy::RETAIN),
       characteristic_value_read_or_write_in_progress_(false) {
   uuid_ = BluetoothAdapterMac::BluetoothUUIDWithCBUUID(
@@ -245,6 +247,7 @@ void BluetoothRemoteGattCharacteristicMac::UnsubscribeFromNotifications(
 void BluetoothRemoteGattCharacteristicMac::DiscoverDescriptors() {
   VLOG(1) << *this << ": Discover descriptors.";
   is_discovery_complete_ = false;
+  ++discovery_pending_count_;
   [GetCBPeripheral()
       discoverDescriptorsForCharacteristic:cb_characteristic_.get()];
 }
@@ -346,7 +349,9 @@ void BluetoothRemoteGattCharacteristicMac::DidUpdateNotificationState(
 
 void BluetoothRemoteGattCharacteristicMac::DidDiscoverDescriptors() {
   DCHECK(!is_discovery_complete_);
+  DCHECK(discovery_pending_count_ > 0);
   VLOG(1) << *this << ": Did discover descriptors.";
+  --discovery_pending_count_;
   std::unordered_set<std::string> descriptor_identifier_to_remove;
   for (const auto& iter : gatt_descriptor_macs_) {
     descriptor_identifier_to_remove.insert(iter.first);
@@ -379,7 +384,7 @@ void BluetoothRemoteGattCharacteristicMac::DidDiscoverDescriptors() {
     gatt_descriptor_macs_.erase(pair_to_remove);
     GetMacAdapter()->NotifyGattDescriptorRemoved(descriptor_to_remove.get());
   }
-  is_discovery_complete_ = true;
+  is_discovery_complete_ = discovery_pending_count_ == 0;
 }
 
 bool BluetoothRemoteGattCharacteristicMac::IsReadable() const {