[sensors] Round off Ambient Light Sensor readouts to the nearest 50 Lux.

There have been some concerns regarding the precision of readouts from the
Ambient Light Sensor. To decrease the entropy, we are rounding off the
illuminance value to the nearest 50 Lux to mitigate the known attack
vectors as summarized in [1].

[1] w3c/ambient-light#13 (comment)

This CL is based on
https://chromium-review.googlesource.com/c/chromium/src/+/1695412 by
Rijubrata Bhaumik and myself.

Bug: 642731, 606766
Change-Id: I85449bdb835e8486b080fc269938b6c96f000c2a

generic-sensor/generic-sensor-tests.js

@@ -246,12 +246,15 @@ function runGenericSensorTests(sensorName,
     assert_true(verificationFunction(expected, sensor2, /*isNull=*/true));
   }, `${sensorName}: sensor reading is correct.`);
 
-  sensor_test(async t => {
+  sensor_test(async (t, sensorProvider) => {
     assert_true(sensorName in self);
     const sensor = new sensorType();
     const sensorWatcher = new EventWatcher(t, sensor, ["reading", "error"]);
     sensor.start();
 
+    const mockSensor = await sensorProvider.getCreatedSensor(sensorName);
+    await mockSensor.setSensorReading(readings);
+
     await sensorWatcher.wait_for("reading");
     const cachedTimeStamp1 = sensor.timestamp;
 
@@ -392,6 +395,7 @@ function runGenericSensorTests(sensorName,
     fastSensor.start();
 
     const mockSensor = await sensorProvider.getCreatedSensor(sensorName);
+    await mockSensor.setSensorReading(readings);
 
     const fastCounter = await new Promise((resolve, reject) => {
       let fastSensorNotifiedCounter = 0;

resources/chromium/generic_sensor_mocks.js

@@ -199,6 +199,19 @@ var GenericSensorTest = (() => {
         this.maxFrequency_ = Math.min(10, this.maxFrequency_);
       }
 
+      // Chromium applies some rounding and other privacy-related measures that
+      // can cause ALS not to report a reading when it has not changed beyond a
+      // certain threshold compared to the previous illuminance value. Make
+      // each reading return a different value that is significantly different
+      // from the previous one when setSensorReading() is not called by client
+      // code (e.g. run_generic_sensor_iframe_tests()).
+      if (type == device.mojom.SensorType.AMBIENT_LIGHT) {
+        this.activeSensors_.get(type).setSensorReading([
+          [window.performance.now() * 100],
+          [(window.performance.now() + 50) * 100]
+        ]);
+      }
+
       const initParams = new device.mojom.SensorInitParams({
         sensor: sensorPtr,
         clientReceiver: mojo.makeRequest(this.activeSensors_.get(type).client_),