[sensors] PlatformSensorProvider implementations are notified when there are no sensor instances

device/generic_sensor/platform_sensor_provider_base.h

Index: device/generic_sensor/platform_sensor_provider_base.h
diff --git a/device/generic_sensor/platform_sensor_provider_base.h b/device/generic_sensor/platform_sensor_provider_base.h
index 2065a3f695fb5836240640da9940bb0dee03ae68..a5da624c4f0e241e00c80110b3e45c72603ed7d8 100644
--- a/device/generic_sensor/platform_sensor_provider_base.h
+++ b/device/generic_sensor/platform_sensor_provider_base.h
@@ -31,6 +31,10 @@ class DEVICE_GENERIC_SENSOR_EXPORT PlatformSensorProviderBase
   // Shared buffer getters.
   mojo::ScopedSharedBufferHandle CloneSharedBufferHandle();
 
+  // Returns 'true' if some of sensor instances produced by this provider are
+  // alive; 'false' otherwise.
+  bool HasSensors() const;
+
  protected:
   PlatformSensorProviderBase();
   virtual ~PlatformSensorProviderBase();
@@ -40,6 +44,10 @@ class DEVICE_GENERIC_SENSOR_EXPORT PlatformSensorProviderBase
                                     mojo::ScopedSharedBufferMapping mapping,
                                     const CreateSensorCallback& callback) = 0;
 
+  // Implementations might override this method to free resources when there
+  // are no sensors left.
+  virtual void AllSensorsRemoved() {}
+
  private:
   friend class PlatformSensor;  // To call RemoveSensor();