[gamepad] Unregister GamepadMonitor at destruction

Previously, GamepadMonitor would only remove itself as a gamepad
consumer if it was active (GamepadStartPolling called with no
corresponding GamepadStopPolling), and would leak if it was
inactive.

This CL ensures GamepadMonitor is always removed if it was ever
added as a gamepad consumer.

BUG=998128

Change-Id: If4f1ac4d1f3b6889acffeb584c2c6dc04b310e31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804836Reviewed-by: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697451}

device/gamepad/gamepad_monitor.cc

@@ -14,10 +14,10 @@
 
 namespace device {
 
-GamepadMonitor::GamepadMonitor() : is_started_(false) {}
+GamepadMonitor::GamepadMonitor() = default;
 
 GamepadMonitor::~GamepadMonitor() {
-  if (is_started_)
+  if (is_registered_consumer_)
     GamepadService::GetInstance()->RemoveConsumer(this);
 }
 
@@ -49,6 +49,7 @@ void GamepadMonitor::OnGamepadButtonOrAxisChanged(uint32_t index,
 void GamepadMonitor::GamepadStartPolling(GamepadStartPollingCallback callback) {
   DCHECK(!is_started_);
   is_started_ = true;
+  is_registered_consumer_ = true;
 
   GamepadService* service = GamepadService::GetInstance();
   service->ConsumerBecameActive(this);

device/gamepad/gamepad_monitor.h

@@ -37,7 +37,12 @@ class DEVICE_GAMEPAD_EXPORT GamepadMonitor : public GamepadConsumer,
 
  private:
   mojo::Remote<mojom::GamepadObserver> gamepad_observer_remote_;
-  bool is_started_;
+
+  // True if this monitor is an active gamepad consumer.
+  bool is_started_ = false;
+
+  // True if this monitor has been registered with the gamepad service.
+  bool is_registered_consumer_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(GamepadMonitor);
 };