Add documentation to the cc scrollbar layer classes

Bug: 1058834
Change-Id: I9fbb4ad17f40860e7864cc01693bfc0bb911a3c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2099342Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749400}

cc/layers/painted_overlay_scrollbar_layer.h

@@ -14,7 +14,9 @@
 namespace cc {
 
 // For composited overlay scrollbars with nine-patch thumb. For overlay
-// scrollbars whose thumb is not nine-patch, use PaintedScrollbarLayer.
+// scrollbars whose thumb is not nine-patch, use PaintedScrollbarLayer or
+// SolidColorScrollbarLayer. In practice, this is used for non-custom
+// overlay scrollbars on Win/Linux.
 class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerBase {
  public:
   std::unique_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl) override;

cc/layers/painted_scrollbar_layer.h

@@ -13,6 +13,10 @@
 
 namespace cc {
 
+// Generic scrollbar layer for cases not covered by PaintedOverlayScrollbarLayer
+// or SolidColorScrollbarLayer. This is not used for CSS-styled scrollbars. In
+// practice, this is used for overlay and non-overlay scrollbars on MacOS, as
+// well as non-overlay scrollbars on Win/Linux.
 class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerBase {
  public:
   std::unique_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl) override;

cc/layers/solid_color_scrollbar_layer.h

@@ -11,6 +11,8 @@
 
 namespace cc {
 
+// A solid color scrollbar that can be fully drawn on the impl thread. In
+// practice, this is used for overlay scrollbars on Android.
 class CC_EXPORT SolidColorScrollbarLayer : public ScrollbarLayerBase {
  public:
   std::unique_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl) override;