Reduce resolution of performance.now to prevent timing attacks

Source/core/timing/PerformanceBase.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) 2012 Intel Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 303 matching lines @@
 
 void PerformanceBase::clearMeasures(const String& measureName)
 {
     if (!m_userTiming)
         m_userTiming = UserTiming::create(this);
     m_userTiming->clearMeasures(measureName);
 }
 
 double PerformanceBase::now() const
 {
-    return 1000.0 * (monotonicallyIncreasingTime() - m_timeOrigin);
+    double nowSeconds = monotonicallyIncreasingTime() - m_timeOrigin;
+    const double resolutionSeconds = 0.000005;
+    return 1000.0 * floor(nowSeconds / resolutionSeconds) * resolutionSeconds;
 }
 
 DEFINE_TRACE(PerformanceBase)
 {
     visitor->trace(m_frameTimingBuffer);
     visitor->trace(m_resourceTimingBuffer);
     visitor->trace(m_userTiming);
     EventTargetWithInlineData::trace(visitor);
 }
 
 } // namespace blink