Fix scrolling perf regression

pixels per mouse_scroll_wheel_tick was being set wrong (should have
been wheel_scroll_factor but was the value divided by dip_scale
was set), hence decreased the scroll rate and caused the regression.
This CL fixes the issue. Now WindowAndroid stores wheel_scroll_factor
(instead of tick multiplier) which is accessed much more often, for
efficiency's sake.

Bug: 821101
Change-Id: I99e81fdfc21ec101a99092c50555be4f429d7a20
Reviewed-on: https://chromium-review.googlesource.com/961503Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543013}

content/browser/renderer_host/render_widget_host_view_android.cc

@@ -444,7 +444,7 @@ int RenderWidgetHostViewAndroid::GetMouseWheelMinimumGranularity() const {
   // many pixels each tick scrolls. This multiplier is specified by device
   // metrics (See WindowAndroid.getMouseWheelScrollFactor) so the minimum
   // granularity will be the size of this tick multiplier.
-  return window->mouse_wheel_tick_multiplier();
+  return window->mouse_wheel_scroll_factor() / view_.GetDipScale();
 }
 
 void RenderWidgetHostViewAndroid::UpdateCursor(const WebCursor& cursor) {

ui/android/event_forwarder.cc

@@ -143,8 +143,9 @@ void EventForwarder::OnMouseWheelEvent(JNIEnv* env,
       0, x, y, 0.0f /* touch_major */, 0.0f /* touch_minor */, 0.0f, 0.0f, 0);
 
   auto* window = view_->GetWindowAndroid();
-  float pixels_per_tick = window ? window->mouse_wheel_tick_multiplier()
-                                 : kDefaultMouseWheelTickMultiplier;
+  float pixels_per_tick =
+      window ? window->mouse_wheel_scroll_factor()
+             : kDefaultMouseWheelTickMultiplier * view_->GetDipScale();
   ui::MotionEventAndroid event(
       env, nullptr, 1.f / view_->GetDipScale(), ticks_x, ticks_y,
       pixels_per_tick, time_ms, 0 /* action */, 1 /* pointer_count */,

ui/android/window_android.cc

@@ -136,9 +136,9 @@ WindowAndroid::WindowAndroid(JNIEnv* env,
       begin_frame_source_(new WindowBeginFrameSource(this)),
       needs_begin_frames_(false) {
   java_window_.Reset(env, obj);
-  mouse_wheel_tick_multiplier_ = scroll_factor > 0
-                                     ? scroll_factor / GetDipScale()
-                                     : kDefaultMouseWheelTickMultiplier;
+  mouse_wheel_scroll_factor_ =
+      scroll_factor > 0 ? scroll_factor
+                        : kDefaultMouseWheelTickMultiplier * GetDipScale();
 }
 
 void WindowAndroid::Destroy(JNIEnv* env, const JavaParamRef<jobject>& obj) {

ui/android/window_android.h

@@ -88,9 +88,7 @@ class UI_ANDROID_EXPORT WindowAndroid : public ViewAndroid {
   // Return whether the specified Android permission can be requested by Chrome.
   bool CanRequestPermission(const std::string& permission);
 
-  float mouse_wheel_tick_multiplier() const {
-    return mouse_wheel_tick_multiplier_;
-  }
+  float mouse_wheel_scroll_factor() const { return mouse_wheel_scroll_factor_; }
 
   static WindowAndroid* CreateForTesting();
 
@@ -120,7 +118,7 @@ class UI_ANDROID_EXPORT WindowAndroid : public ViewAndroid {
   std::unique_ptr<WindowBeginFrameSource> begin_frame_source_;
   bool needs_begin_frames_;
   std::list<base::Closure> vsync_complete_callbacks_;
-  float mouse_wheel_tick_multiplier_;
+  float mouse_wheel_scroll_factor_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowAndroid);
 };