[PercentBasedScrolling] Make flag accessible to LayerTreeHostImpl.

This CL adds the plumbing to make features::kPercentBasedScrolling
accessible to LayerTreeHostImpl via LayerTreeSettings.

Bug: 1062163
Change-Id: Ic8676f3c1d76533fcc3639de68ece2763baae711
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107153Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#751091}

cc/input/scrollbar_controller.cc

@@ -615,6 +615,9 @@ int ScrollbarController::GetScrollDeltaForScrollbarPart(
     const ScrollbarPart scrollbar_part,
     const bool shift_modifier) {
   int scroll_delta = 0;
+  if (layer_tree_host_impl_->settings().percent_based_scrolling) {
+    // TODO(arakeri): Implement percent based deltas.
+  }
 
   switch (scrollbar_part) {
     case ScrollbarPart::BACK_BUTTON:

cc/trees/layer_tree_settings.h

@@ -142,6 +142,9 @@ class CC_EXPORT LayerTreeSettings {
   // on the compositor thread.
   bool compositor_threaded_scrollbar_scrolling = true;
 
+  // If enabled, the scroll deltas will be a percentage of the target scroller.
+  bool percent_based_scrolling = false;
+
   // Determines whether animated scrolling is supported. If true, and the
   // incoming gesture scroll is of a type that would normally be animated (e.g.
   // coarse granularity scrolls like those coming from an external mouse wheel),

content/renderer/render_widget.cc

@@ -2688,6 +2688,8 @@ cc::LayerTreeSettings RenderWidget::GenerateLayerTreeSettings(
   cc::LayerTreeSettings settings;
 
   settings.use_zoom_for_dsf = compositor_deps->IsUseZoomForDSFEnabled();
+  settings.percent_based_scrolling =
+      base::FeatureList::IsEnabled(features::kPercentBasedScrolling);
   settings.compositor_threaded_scrollbar_scrolling =
       base::FeatureList::IsEnabled(
           features::kCompositorThreadedScrollbarScrolling);

ui/compositor/compositor.cc

@@ -216,6 +216,10 @@ Compositor::Compositor(const viz::FrameSinkId& frame_sink_id,
     settings.compositor_threaded_scrollbar_scrolling = true;
   }
 
+  if (base::FeatureList::IsEnabled(features::kPercentBasedScrolling)) {
+    settings.percent_based_scrolling = true;
+  }
+
 #if DCHECK_IS_ON()
   if (command_line->HasSwitch(cc::switches::kLogOnUIDoubleBackgroundBlur))
     settings.log_on_ui_double_background_blur = true;