Add RELATIVE_ORIENTATION sensor implementation on macOS to //device/generic_sensor

device/generic_sensor/README.md

 # Sensors
 
 `device/generic_sensor` contains the platform-specific parts of the Sensor APIs
 implementation.
 
 Sensors Mojo interfaces are defined in the `public/interfaces` subdirectory.
 
 The JS bindings are implemented in `third_party/WebKit/Source/modules/sensor`.
 
 
 ## Platform Support
 
 Support for the SensorTypes defined by the Mojo interface is summarized in this
 table. An empty cell indicates that the sensor type is not supported on that
 platform.
 
 | SensorType           | Android                   | Linux          | macOS              | Windows                                   |
 | -------------------- | ------------------------- | -------------- | ------------------ | ----------------------------------------- |
 | AMBIENT_LIGHT        | TYPE_LIGHT                | in_illuminance | AppleLMUController | SENSOR_TYPE_AMBIENT_LIGHT                 |
 | PROXIMITY            |                           |                |                    |                                           |
 | ACCELEROMETER        | TYPE_ACCELEROMETER        | in_accel       | SMCMotionSensor    | SENSOR_TYPE_ACCELEROMETER_3D              |
 | LINEAR_ACCELEROMETER | TYPE_LINEAR_ACCELEROMETER |                |                    |                                           |
 | GYROSCOPE            | TYPE_GYROSCOPE            | in_anglvel     |                    | SENSOR_TYPE_GYROMETER_3D                  |
 | MAGNETOMETER         | TYPE_MAGNETIC_FIELD       | in_magn        |                    | SENSOR_TYPE_COMPASS_3D                    |
 | PRESSURE             |                           |                |                    |                                           |
 | ABSOLUTE_ORIENTATION | TYPE_ROTATION_VECTOR      |                |                    | SENSOR_TYPE_AGGREGATED_DEVICE_ORIENTATION |
-| RELATIVE_ORIENTATION | TYPE_GAME_ROTATION_VECTOR |                |                    |                                           |
+| RELATIVE_ORIENTATION | TYPE_GAME_ROTATION_VECTOR |                | ACCELEROMETER      |                                           |
 
 ### Android
 
 Sensors are implemented by passing through values provided by the
 [Sensor](https://developer.android.com/reference/android/hardware/Sensor.html)
 class. The values in the "Android" column of the table above correspond to the
 integer constants from the android.hardware.Sensor used to provide data for a
 SensorType.
 
 ### Linux (and Chrome OS)
 
 Sensors are implemented by reading values from the IIO subsystem. The values in
 the "Linux" column of the table above are the prefix of the sysfs files Chrome
 searches for to provide data for a SensorType.
 
 ### macOS
 
 On this platform there is limited support for sensors. The AMBIENT_LIGHT sensor
 type is provided by interpreting the value that can be read from the LMU. The
 ACCELEROMETER sensor type is provided by interpreting the value that can be read
-from the SMCMotionSensor.
+from the SMCMotionSensor. The RELATIVE_ORIENTATION sensor type is provided by
+interpreting the value that can be read from the ACCELEROMETER.
 
 ### Windows
 
 Sensors are implemented by passing through values provided by the
 [Sensor API](https://msdn.microsoft.com/en-us/library/windows/desktop/dd318953(v=vs.85).aspx).
 The values in the "Windows" column of the table above correspond to the names of
 the sensor type GUIDs used to provide data for a SensorType.
 
 ## Testing
 
 Sensors platform unit tests are located in the current directory and its
 subdirectories.
 
 The sensors unit tests file for Android is
 `android/junit/src/org/chromium/device/sensors/PlatformSensorAndProviderTest.java`.
 
 Sensors browser tests are located in `content/test/data/generic_sensor`.
 
 
 ## Design Documents
 
 Please refer to the [design documentation](https://docs.google.com/document/d/1Ml65ZdW5AgIsZTszk4mD_ohr40pcrdVFOIf0ZtWxDv0)
 for more details.