Add UMA metric to track the time saved on making events passive before pageload

third_party/WebKit/Source/core/input/TouchEventManager.cpp

Index: third_party/WebKit/Source/core/input/TouchEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/TouchEventManager.cpp b/third_party/WebKit/Source/core/input/TouchEventManager.cpp
index 23c662db3de37c89c572522c479f0b4547ae49c8..85c7294e600e2758ed7bf99b753b7387305fc6b8 100644
--- a/third_party/WebKit/Source/core/input/TouchEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/TouchEventManager.cpp
@@ -16,7 +16,7 @@
 #include "core/page/Page.h"
 #include "platform/Histogram.h"
 #include "platform/PlatformTouchEvent.h"
-
+#include "wtf/CurrentTime.h"
 
 
 namespace blink {
@@ -243,13 +243,20 @@ WebInputEventResult TouchEventManager::dispatchTouchEvents(
                 DEFINE_STATIC_LOCAL(EnumerationHistogram, rootDocumentListenerHistogram, ("Event.Touch.TargetAndDispatchResult", TouchTargetAndDispatchResultTypeMax));
                 rootDocumentListenerHistogram.count(toTouchTargetHistogramValue(eventTarget, domDispatchResult));
 
-                // Count the handled touch starts and first touch moves before and after the page is fully loaded respectively.
+                // 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);
                 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);
+
+                    DEFINE_STATIC_LOCAL(CustomCountHistogram, eventLatencyBeforePageLoadHistogram, ("Event.Touch.TouchLatencyBeforePageLoad", 1, 100000000, 50));
+                    eventLatencyBeforePageLoadHistogram.count(latencyInMicros);
                 }
             }
             eventResult = EventHandler::mergeEventResult(eventResult,