Revert of Remove a frame of delay on main-thread FlingStart. (patchset #1 id:1...

Revert of Remove a frame of delay on main-thread FlingStart. (patchset #1 id:1 of https://codereview.chromium.org/2908073008/ )

Reason for revert:
Possible cause of flakiness in fast/events/touch/gesture/pad-gesture-fling.html ; see e.g. https://luci-milo.appspot.com/buildbot/chromium.webkit/WebKit%20Mac10.11%20%28dbg%29/8992 .

Original issue's description:
> Remove a frame of delay on main-thread FlingStart.
>
> Main-thread flings were waiting for the first frame of animation to
> populate the start timestamp, and only on the second frame would they
> start animating.  But the start timestamp is included in the FlingStart
> event, so there's no need for this (perhaps it was written that way for
> historical reasons when the event timestamp wasn't populated properly).
>
> This problem was extremely noticeable in prototype of middle-click autoscroll
> that issues a FlingStart on every MouseMove to change velocity, but
> the fix should also benefit touchscreen and touchpad flings slightly.
>
> BUG=727964
>
> Review-Url: https://codereview.chromium.org/2908073008
> Cr-Commit-Position: refs/heads/master@{#475984}
> Committed: https://chromium.googlesource.com/chromium/src/+/3562f1b7410b884e07bbd2fc75cfee7e96ed2a41

TBR=jbroman@chromium.org,dtapuska@chromium.org,aelias@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=727964

Review-Url: https://codereview.chromium.org/2917823003
Cr-Commit-Position: refs/heads/master@{#476181}

third_party/WebKit/LayoutTests/fast/scroll-behavior/scroll-customization/touch-scroll-customization.html

@@ -204,14 +204,12 @@ if ('ScrollState' in window) {
       assert_equals(document.scrollingElement.scrollTop, scrollingElementTop, "For scrollingElement on step " + step);
     };
 
-    assertScrollTops(0, 0, 0, 0);
-
     var frame_actions = [
       function() {
         eventSender.gestureFlingStart(10, 10, -1000000, -1000000, "touchscreen");
       },
       flingTest.step_func(function() {
-        assertScrollTops(100, 0, 0, 1);
+        assertScrollTops(0, 0, 0, 1);
       }),
       flingTest.step_func(function() {
         assertScrollTops(100, 0, 0, 2);

third_party/WebKit/Source/platform/exported/WebActiveGestureAnimation.cpp

@@ -32,13 +32,21 @@
 
 namespace blink {
 
+std::unique_ptr<WebActiveGestureAnimation>
+WebActiveGestureAnimation::CreateAtAnimationStart(
+    std::unique_ptr<WebGestureCurve> curve,
+    WebGestureCurveTarget* target) {
+  return WTF::WrapUnique(
+      new WebActiveGestureAnimation(std::move(curve), target, 0, true));
+}
+
 std::unique_ptr<WebActiveGestureAnimation>
 WebActiveGestureAnimation::CreateWithTimeOffset(
     std::unique_ptr<WebGestureCurve> curve,
     WebGestureCurveTarget* target,
     double start_time) {
-  return WTF::WrapUnique(
-      new WebActiveGestureAnimation(std::move(curve), target, start_time));
+  return WTF::WrapUnique(new WebActiveGestureAnimation(std::move(curve), target,
+                                                       start_time, false));
 }
 
 WebActiveGestureAnimation::~WebActiveGestureAnimation() {}
@@ -46,10 +54,18 @@ WebActiveGestureAnimation::~WebActiveGestureAnimation() {}
 WebActiveGestureAnimation::WebActiveGestureAnimation(
     std::unique_ptr<WebGestureCurve> curve,
     WebGestureCurveTarget* target,
-    double start_time)
-    : start_time_(start_time), curve_(std::move(curve)), target_(target) {}
+    double start_time,
+    bool waiting_for_first_tick)
+    : start_time_(start_time),
+      waiting_for_first_tick_(waiting_for_first_tick),
+      curve_(std::move(curve)),
+      target_(target) {}
 
 bool WebActiveGestureAnimation::Animate(double time) {
+  if (waiting_for_first_tick_) {
+    start_time_ = time;
+    waiting_for_first_tick_ = false;
+  }
   // All WebGestureCurves assume zero-based time, so we subtract
   // the animation start time before passing to the curve.
   return curve_->Apply(time - start_time_, target_);

third_party/WebKit/Source/platform/exported/WebActiveGestureAnimation.h

@@ -46,6 +46,9 @@ class PLATFORM_EXPORT WebActiveGestureAnimation {
   WTF_MAKE_NONCOPYABLE(WebActiveGestureAnimation);
 
  public:
+  static std::unique_ptr<WebActiveGestureAnimation> CreateAtAnimationStart(
+      std::unique_ptr<WebGestureCurve>,
+      WebGestureCurveTarget*);
   static std::unique_ptr<WebActiveGestureAnimation> CreateWithTimeOffset(
       std::unique_ptr<WebGestureCurve>,
       WebGestureCurveTarget*,
@@ -58,9 +61,11 @@ class PLATFORM_EXPORT WebActiveGestureAnimation {
   // Assumes a valid WebGestureCurveTarget that outlives the animation.
   WebActiveGestureAnimation(std::unique_ptr<WebGestureCurve>,
                             WebGestureCurveTarget*,
-                            double start_time);
+                            double start_time,
+                            bool waiting_for_first_tick);
 
   double start_time_;
+  bool waiting_for_first_tick_;
   std::unique_ptr<WebGestureCurve> curve_;
   WebGestureCurveTarget* target_;
 };

third_party/WebKit/Source/web/WebViewImpl.cpp

@@ -698,8 +698,8 @@ WebInputEventResult WebViewImpl::HandleGestureEvent(
                             event.data.fling_start.velocity_y),
               WebSize());
       DCHECK(fling_curve);
-      gesture_animation_ = WebActiveGestureAnimation::CreateWithTimeOffset(
-          std::move(fling_curve), this, event.TimeStampSeconds());
+      gesture_animation_ = WebActiveGestureAnimation::CreateAtAnimationStart(
+          std::move(fling_curve), this);
       MainFrameImpl()->FrameWidget()->ScheduleAnimation();
       event_result = WebInputEventResult::kHandledSystem;