[sensors] Ambient light sensor bindings implementation

third_party/WebKit/Source/modules/sensor/SensorReading.cpp

Index: third_party/WebKit/Source/modules/sensor/SensorReading.cpp
diff --git a/third_party/WebKit/Source/modules/sensor/SensorReading.cpp b/third_party/WebKit/Source/modules/sensor/SensorReading.cpp
index 12c14881bc9af7be3a74f93e364ea500abd8c641..061711983e3da38fbe6c470411a3aa26362b08e7 100644
--- a/third_party/WebKit/Source/modules/sensor/SensorReading.cpp
+++ b/third_party/WebKit/Source/modules/sensor/SensorReading.cpp
@@ -14,7 +14,6 @@ namespace blink {
 SensorReading::SensorReading(SensorProxy* sensorProxy)
     : m_sensorProxy(sensorProxy)
 {
-    DCHECK(m_sensorProxy);
 }
 
 DEFINE_TRACE(SensorReading)
@@ -31,6 +30,15 @@ DOMHighResTimeStamp SensorReading::timeStamp(ScriptState* scriptState) const
     Performance* performance = DOMWindowPerformance::performance(*window);
     DCHECK(performance);
 
+    if (!m_sensorProxy) {
+        // In cases when SensorReading derived classes are constructed from JS
+        // side, e.g. to create syntetic SensorReadingEvent for testing
+        // purposes, |m_sensorProxy| will be null and SensorReading.timeStamp
+        // would return current DOMHighResTimeStamp, while reading value should
+        // be provided by derived classes.
+        return performance->now();
+    }
+
     return performance->monotonicTimeToDOMHighResTimeStamp(m_sensorProxy->reading().timestamp);
 }