[iOS] Fullscreen animation polish for fast scoll events.

This CL updates fullscreen to:
- not perform an animation if a scroll ends when the toolbar is fully
  visible or hidden.
- scale the scroll end adjustment animation duration to the fraction
  of progress change required.

Bug: 796597
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I8939fbdd93735155e41a9ed1e2d19d1b72902b4a
Reviewed-on: https://chromium-review.googlesource.com/841535Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Louis Romero <lpromero@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526026}

ios/chrome/browser/ui/fullscreen/fullscreen_mediator.mm

@@ -9,6 +9,7 @@
 #import "ios/chrome/browser/ui/fullscreen/fullscreen_controller_observer.h"
 #import "ios/chrome/browser/ui/fullscreen/fullscreen_model.h"
 #import "ios/chrome/browser/ui/fullscreen/fullscreen_scroll_end_animator.h"
+#include "ios/chrome/browser/ui/ui_util.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
@@ -64,8 +65,11 @@ void FullscreenMediator::FullscreenModelScrollEventEnded(
     FullscreenModel* model) {
   DCHECK_EQ(model_, model);
   DCHECK(!animator_);
-  animator_ = [[FullscreenScrollEndAnimator alloc]
-      initWithStartProgress:model_->progress()];
+  CGFloat progress = model_->progress();
+  if (AreCGFloatsEqual(progress, 0.0) || AreCGFloatsEqual(progress, 1.0))
+    return;
+  animator_ =
+      [[FullscreenScrollEndAnimator alloc] initWithStartProgress:progress];
   [animator_ addCompletion:^(UIViewAnimatingPosition finalPosition) {
     DCHECK_EQ(finalPosition, UIViewAnimatingPositionEnd);
     model_->AnimationEndedWithProgress(

ios/chrome/browser/ui/fullscreen/fullscreen_scroll_end_animator.mm

@@ -33,8 +33,15 @@
   UICubicTimingParameters* timingParams = [[UICubicTimingParameters alloc]
       initWithControlPoint1:CGPointMake(0.0, 0.0)
               controlPoint2:CGPointMake(0.2, 0.1)];
-  self = [super initWithDuration:ios::material::kDuration1
-                timingParameters:timingParams];
+  DCHECK_GE(startProgress, 0.0);
+  DCHECK_LE(startProgress, 1.0);
+  CGFloat finalProgress = roundf(startProgress);
+  // Scale the duration by the progress delta traversed in this animation.
+  // Since |finalProgress - startProgress| <= 0.5, the delta is multiplied by
+  // 2.0.
+  NSTimeInterval duration =
+      2.0 * fabs(finalProgress - startProgress) * ios::material::kDuration1;
+  self = [super initWithDuration:duration timingParameters:timingParams];
   if (self) {
     DCHECK_GE(startProgress, 0.0);
     DCHECK_LE(startProgress, 1.0);