[CompositorThreadedScrollbarScrolling] Turn feature on by default.

This CL turns the "Compositor Threaded Scrollbar Scrolling" on by
default in master. The details about this feature can be found in
the following links:

Design doc: https://docs.google.com/document/d/1JqykSXnCkqwA1E3bUhhIi-IgEvM9HZdKtIu_S4Ncm6o
Demo page: https://rahul8805.github.io/DemoPages/BouncyMoon.html
Lightning talk: https://www.youtube.com/watch?v=FOCHCuGA_MA&t=1150s
input-dev thread: https://groups.google.com/a/chromium.org/forum/#!topic/input-dev/6ACOSDoAik4

Bug: 148501
Change-Id: Iec2e4de5c79e94815629be37c5bbe4ed435e106b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2048418Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#745266}

cc/trees/layer_tree_settings.h

@@ -140,7 +140,7 @@ class CC_EXPORT LayerTreeSettings {
 
   // Determines whether mouse interactions on composited scrollbars are handled
   // on the compositor thread.
-  bool compositor_threaded_scrollbar_scrolling = false;
+  bool compositor_threaded_scrollbar_scrolling = true;
 
   // Determines whether animated scrolling is supported. If true, and the
   // incoming gesture scroll is of a type that would normally be animated (e.g.

ui/base/ui_base_features.cc

@@ -79,7 +79,7 @@ const base::Feature kUiCompositorScrollWithLayers = {
 // Enables compositor threaded scrollbar scrolling by mapping pointer events to
 // gesture events.
 const base::Feature kCompositorThreadedScrollbarScrolling = {
-    "CompositorThreadedScrollbarScrolling", base::FEATURE_DISABLED_BY_DEFAULT};
+    "CompositorThreadedScrollbarScrolling", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables the use of a touch fling curve that is based on the behavior of
 // native apps on Windows.