Reland of [sensors] Ambient light sensor bindings implementation

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/sensor/SensorReading.h"
 
 #include "core/dom/ExecutionContext.h"
 #include "core/frame/LocalDOMWindow.h"
 #include "core/timing/DOMWindowPerformance.h"
 #include "core/timing/Performance.h"
 
 namespace blink {
 
 SensorReading::SensorReading(SensorProxy* sensorProxy)
     : m_sensorProxy(sensorProxy)
 {
-    DCHECK(m_sensorProxy);
 }
 
+SensorReading::~SensorReading() = default;
+
 DEFINE_TRACE(SensorReading)
 {
     visitor->trace(m_sensorProxy);
 }
 
 DOMHighResTimeStamp SensorReading::timeStamp(ScriptState* scriptState) const
 {
     LocalDOMWindow* window = scriptState->domWindow();
     if (!window)
         return 0.0;
 
     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);
 }
 
 } // namespace blink