Fix memory leak in extensions::api::SerialGetDevicesFunction

This change resets the SerialDeviceEnumeratorPtr held by this class
after enumeration is complete. This breaks the reference cycle between
these two objects, allowing them both to be freed.

Bug: 898177
Change-Id: I874c62f89f8bb59a2c41f8c40afd537404f4580b
Reviewed-on: https://chromium-review.googlesource.com/c/1311533Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604415}

extensions/browser/api/serial/serial_api.cc

@@ -102,6 +102,7 @@ void SerialGetDevicesFunction::OnGotDevices(
       serial::GetDevices::Results::Create(
           mojo::ConvertTo<std::vector<serial::DeviceInfo>>(devices));
   Respond(ArgumentList(std::move(results)));
+  enumerator_.reset();
 }
 
 SerialConnectFunction::SerialConnectFunction() {}