[DeviceLight] Ensure to round lux value to avoid fingerprinting risk.

third_party/WebKit/Source/modules/device_light/DeviceLightDispatcher.cpp

Index: third_party/WebKit/Source/modules/device_light/DeviceLightDispatcher.cpp
diff --git a/third_party/WebKit/Source/modules/device_light/DeviceLightDispatcher.cpp b/third_party/WebKit/Source/modules/device_light/DeviceLightDispatcher.cpp
index 1629d9bce07305ed2901eb4b4b8a0a80022ef78a..f7d241748d4c93acc5816da64936a95cc4b42108 100644
--- a/third_party/WebKit/Source/modules/device_light/DeviceLightDispatcher.cpp
+++ b/third_party/WebKit/Source/modules/device_light/DeviceLightDispatcher.cpp
@@ -4,9 +4,21 @@
 
 #include "modules/device_light/DeviceLightDispatcher.h"
 
+#include <math.h>
+
 #include "modules/device_light/DeviceLightController.h"
 #include "public/platform/Platform.h"
 
+namespace {
+double EnsureRoundedLuxValue(double lux) {
+  // Make sure to round the lux value to nearest integer, to
+  // avoid too precise values and hence reduce fingerprinting risk.
+  // The special case when the lux value is infinity (no data can be
+  // provided) is simply returned as is.
+  return isinf(lux) ? lux : std::round(lux);

[palmer, 2017/03/23 19:00:19]

Apologies in advance for the drive-by. :)

A rounded double is still a high-precision value, though. I'd really like to get
it lower if possible, like down to 4 bits of precision. (Or whatever small range
still suits the intended applications.)

[timvolodine, 2017/04/20 01:34:30]

Acknowledged.

+}
+}  // namespace
+
 namespace blink {
 
 DeviceLightDispatcher& DeviceLightDispatcher::instance() {
@@ -33,8 +45,11 @@ void DeviceLightDispatcher::stopListening() {
 }
 
 void DeviceLightDispatcher::didChangeDeviceLight(double value) {
-  m_lastDeviceLightData = value;
-  notifyControllers();
+  double newValue = EnsureRoundedLuxValue(value);
+  if (m_lastDeviceLightData != newValue) {
+    m_lastDeviceLightData = newValue;
+    notifyControllers();
+  }
 }
 
 double DeviceLightDispatcher::latestDeviceLightData() const {