Notify developers of inefficiency to be fixed.

When BluetoothAdapter::Observer::GattCharacteristicValueChanged() is
called WebBluetoothServiceImpl notifies all
blink::BluetoothRemoteGattCharacteristic objects even if
startNotifications() has not been called or there are no listeners. This
is inefficient.

We have decided not to fix this at the moment but we wanted to put a
comment in the code so if anyone is working on that function in the
future they could be aware of this inefficiency and fix it if the change
becomes more straightforward.

Bug: 541390
Change-Id: Icfc54cb17ac76b856effbfac97af98574a6ee17a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724588Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: James Hollyer <jameshollyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682070}

content/browser/bluetooth/web_bluetooth_service_impl.cc

@@ -561,6 +561,9 @@ void WebBluetoothServiceImpl::GattCharacteristicValueChanged(
     return;
   }
 
+  // TODO(crbug.com/541390): Don't send notifications when they haven't been
+  // requested by the client.
+
   // On Chrome OS and Linux, GattCharacteristicValueChanged is called before the
   // success callback for ReadRemoteCharacteristic is called, which could result
   // in an event being fired before the readValue promise is resolved.