[Android] Use oldest historical event time as hardware timestamp

On other platforms we recored hardware timestamp to measure end-to-end
latency, however on Android the 'hardware timestamp' we got is actually
a resample (or latest) timestamp due to it's event batching mechanism.

This patch switched to use the oldest historical event time to match
other platforms and for better measuring of end-to-end performance.

Expecting to see regressions in UMA similar to
'Event.Latency.ScrollUpdate.Touch.TimeToScrollUpdateSwapBegin2',
e.g. Probably a 10ms ~ 20ms increase.

Note: This patch has no effect on telemetry tests.

Bug: 737379
Change-Id: I3bb4349ec1a47dc497817d70c2c5a64ed40f910a
Reviewed-on: https://chromium-review.googlesource.com/577962Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Alexandre Elias <aelias@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Chong Zhang <chongz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488447}

ui/android/java/src/org/chromium/ui/base/EventForwarder.java

@@ -78,6 +78,12 @@ public class EventForwarder {
 
         TraceEvent.begin("sendTouchEvent");
         try {
+            // Android may batch multiple events together for efficiency. We
+            // want to use the oldest event time as hardware time stamp.
+            final long oldestEventTime = event.getHistorySize() > 0
+                    ? event.getHistoricalEventTime(0)
+                    : event.getEventTime();
+
             int eventAction = event.getActionMasked();
 
             eventAction = SPenSupport.convertSPenEventAction(eventAction);
@@ -107,7 +113,7 @@ public class EventForwarder {
             }
 
             final boolean consumed = nativeOnTouchEvent(mNativeEventForwarder, event,
-                    event.getEventTime(), eventAction, pointerCount, event.getHistorySize(),
+                    oldestEventTime, eventAction, pointerCount, event.getHistorySize(),
                     event.getActionIndex(), event.getX(), event.getY(),
                     pointerCount > 1 ? event.getX(1) : 0, pointerCount > 1 ? event.getY(1) : 0,
                     event.getPointerId(0), pointerCount > 1 ? event.getPointerId(1) : -1,