Add an additional constructor on ScrollView to scroll with layers.

Currently layer scrolling is only enabled on mac by default.

Providing a constructor arg allows users of ScrollView to enable layer
scrolling regardless of platform.

This will initially be used for the ScrollView container that holds the
tabstrip for Tab Scrolling.

Bug: 1129675

Change-Id: Ie1a34d45eac952fcaf08daa313ee15bb36aa58ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416943Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810690}

chrome/browser/ui/views/frame/tab_strip_region_view.cc

@@ -25,7 +25,8 @@ TabStripRegionView::TabStripRegionView(std::unique_ptr<TabStrip> tab_strip) {
                           base::Unretained(this)));
   if (base::FeatureList::IsEnabled(features::kScrollableTabStrip)) {
     views::ScrollView* tab_strip_scroll_container =
-        AddChildView(std::make_unique<views::ScrollView>());
+        AddChildView(std::make_unique<views::ScrollView>(
+            views::ScrollView::ScrollWithLayers::kEnabled));
     tab_strip_scroll_container->SetBackgroundColor(base::nullopt);
     tab_strip_scroll_container->SetHideHorizontalScrollBar(true);
     tab_strip_container_ = tab_strip_scroll_container;

ui/views/controls/scroll_view.cc

@@ -162,11 +162,17 @@ class ScrollView::Viewport : public View {
 };
 
 ScrollView::ScrollView()
+    : ScrollView(base::FeatureList::IsEnabled(
+                     ::features::kUiCompositorScrollWithLayers)
+                     ? ScrollWithLayers::kEnabled
+                     : ScrollWithLayers::kDisabled) {}
+
+ScrollView::ScrollView(ScrollWithLayers scroll_with_layers)
     : horiz_sb_(PlatformStyle::CreateScrollBar(true)),
       vert_sb_(PlatformStyle::CreateScrollBar(false)),
       corner_view_(std::make_unique<ScrollCornerView>()),
-      scroll_with_layers_enabled_(base::FeatureList::IsEnabled(
-          ::features::kUiCompositorScrollWithLayers)) {
+      scroll_with_layers_enabled_(scroll_with_layers ==
+                                  ScrollWithLayers::kEnabled) {
   SetNotifyEnterExitOnChild(true);
 
   // Since |contents_viewport_| is accessed during the AddChildView call, make

ui/views/controls/scroll_view.h

@@ -47,8 +47,16 @@ class VIEWS_EXPORT ScrollView : public View, public ScrollBarController {
  public:
   METADATA_HEADER(ScrollView);
 
+  // Indicates whether or not scroll view is initialized with layer-scrolling.
+  enum class ScrollWithLayers { kDisabled, kEnabled };
+
   ScrollView();
 
+  // Additional constructor for overriding scrolling as defined by
+  // |kUiCompositorScrollWithLayers|. See crbug.com/873923 for more details on
+  // enabling by default this for all platforms.
+  explicit ScrollView(ScrollWithLayers scroll_with_layers);
+
   ~ScrollView() override;
 
   // Creates a ScrollView with a theme specific border.