Use monotonic time to record the touch event latency in Blink

third_party/WebKit/Source/core/input/TouchEventManager.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 "core/input/TouchEventManager.h"
 
 #include "core/dom/Document.h"
 #include "core/events/TouchEvent.h"
 #include "core/frame/Deprecation.h"
 #include "core/frame/EventHandlerRegistry.h"
@@ skipping 175 matching lines @@
                 eventName, touchEventTarget->toNode()->document().domWindow(),
                 event.getModifiers(), event.cancelable(), event.causesScrollingIfUncanceled(), touchStartOrFirstTouchMove, event.timestamp());
 
             DispatchEventResult domDispatchResult = touchEventTarget->dispatchEvent(touchEvent);
 
             // Only report for top level documents with a single touch on
             // touch-start or the first touch-move.
             if (touchStartOrFirstTouchMove && touchInfos.size() == 1 && event.cancelable() && m_frame->isMainFrame()) {
 
                 // Record the disposition and latency of touch starts and first touch moves before and after the page is fully loaded respectively.
-                int64_t latencyInMicros = static_cast<int64_t>((currentTime() - event.timestamp()) * 1000000.0);
+                int64_t latencyInMicros = static_cast<int64_t>((monotonicallyIncreasingTime() - event.timestamp()) * 1000000.0);
                 if (m_frame->document()->isLoadCompleted()) {
                     DEFINE_STATIC_LOCAL(EnumerationHistogram, touchDispositionsAfterPageLoadHistogram, ("Event.Touch.TouchDispositionsAfterPageLoad", TouchEventDispatchResultTypeMax));
                     touchDispositionsAfterPageLoadHistogram.count((domDispatchResult != DispatchEventResult::NotCanceled) ? HandledTouches : UnhandledTouches);
 
                     DEFINE_STATIC_LOCAL(CustomCountHistogram, eventLatencyAfterPageLoadHistogram, ("Event.Touch.TouchLatencyAfterPageLoad", 1, 100000000, 50));
                     eventLatencyAfterPageLoadHistogram.count(latencyInMicros);
                 } else {
                     DEFINE_STATIC_LOCAL(EnumerationHistogram, touchDispositionsBeforePageLoadHistogram, ("Event.Touch.TouchDispositionsBeforePageLoad", TouchEventDispatchResultTypeMax));
                     touchDispositionsBeforePageLoadHistogram.count((domDispatchResult != DispatchEventResult::NotCanceled) ? HandledTouches : UnhandledTouches);
 
@@ skipping 333 matching lines @@
         feature = UseCounter::TouchEndDuringScrollUserGestureUtilized;
         break;
     default:
         NOTREACHED();
         return;
     }
     Deprecation::countDeprecation(m_frame, feature);
 }
 
 } // namespace blink