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

device/generic_sensor/platform_sensor_provider_base.cc

Index: device/generic_sensor/platform_sensor_provider_base.cc
diff --git a/device/generic_sensor/platform_sensor_provider_base.cc b/device/generic_sensor/platform_sensor_provider_base.cc
index 8224964d939b5fc271acd937ba8c51aff0ab0dde..c3f30ceb648966f50954ee5717d6e42ac4686551 100644
--- a/device/generic_sensor/platform_sensor_provider_base.cc
+++ b/device/generic_sensor/platform_sensor_provider_base.cc
@@ -79,8 +79,10 @@ void PlatformSensorProviderBase::RemoveSensor(mojom::SensorType type) {
   DCHECK(ContainsKey(sensor_map_, type));
   sensor_map_.erase(type);
 
-  if (sensor_map_.empty())
+  if (sensor_map_.empty()) {
+    AllSensorsRemoved();
     shared_buffer_handle_.reset();
+  }
 }
 
 mojo::ScopedSharedBufferHandle
@@ -91,6 +93,11 @@ PlatformSensorProviderBase::CloneSharedBufferHandle() {
       mojo::SharedBufferHandle::AccessMode::READ_ONLY);
 }
 
+bool PlatformSensorProviderBase::HasSensors() const {
+  DCHECK(CalledOnValidThread());
+  return !sensor_map_.empty();
+}
+
 void PlatformSensorProviderBase::NotifySensorCreated(
     mojom::SensorType type,
     scoped_refptr<PlatformSensor> sensor) {