Use monotonic time instead of current time in autoscroll controller

Use of a wall clock to drive an animation is dangerous. We should be using the monotinic time.

BUG=645098

Review-Url: https://codereview.chromium.org/2326643002
Cr-Commit-Position: refs/heads/master@{#417365}

third_party/WebKit/Source/core/page/AutoscrollController.cpp

@@ -233,8 +233,6 @@ bool AutoscrollController::panScrollInProgress() const
 }
 #endif
 
-// FIXME: This would get get better animation fidelity if it used the monotonicFrameBeginTime instead
-// of WTF::currentTime().
 void AutoscrollController::animate(double)
 {
     if (!m_autoscrollLayoutObject) {
@@ -244,7 +242,7 @@ void AutoscrollController::animate(double)
 
     switch (m_autoscrollType) {
     case AutoscrollForDragAndDrop:
-        if (WTF::currentTime() - m_dragAndDropAutoscrollStartTime > autoscrollDelay)
+        if (WTF::monotonicallyIncreasingTime() - m_dragAndDropAutoscrollStartTime > autoscrollDelay)
             m_autoscrollLayoutObject->autoscroll(m_dragAndDropAutoscrollReferencePosition);
         break;
     case AutoscrollForSelection: