Add class-level comments on PaintLayerCompositor and ScrollingCoordinator.

Bug: 740070
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I9c3208a36dd577634c7f74f5ccf63466c8b67f44
Reviewed-on: https://chromium-review.googlesource.com/578653Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488665}

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h

@@ -60,12 +60,21 @@ enum CompositingStateTransitionType {
   kRemoveFromSquashingLayer
 };
 
-// PaintLayerCompositor manages the hierarchy of
-// composited Layers. It determines which Layers
-// become compositing, and creates and maintains a hierarchy of
-// GraphicsLayers based on the Layer painting order.
+// PaintLayerCompositor maintains document-level compositing state and is the
+// entry point of the "compositing update" lifecycle stage.  There is one PLC
+// per LayoutView.
 //
-// There is one PaintLayerCompositor per LayoutView.
+// The compositing update, implemented by PaintLayerCompositor and friends,
+// decides for each PaintLayer whether it should get a CompositedLayerMapping,
+// and asks each CLM to set up its GraphicsLayers.
+//
+// When root layer scrolling is disabled, PaintLayerCompositor also directly
+// manages GraphicsLayers related to FrameView scrolling.  See VisualViewport.h
+// for a diagram of how these layers are wired.
+//
+// When root layer scrolling is enabled, PaintLayerCompositor does not create
+// any of its own GraphicsLayers.  Instead the LayoutView's CLM is wired
+// directly to the scroll layer of the visual viewport.
 //
 // In Slimming Paint v2, PaintLayerCompositor will be eventually replaced by
 // PaintArtifactCompositor.

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.h

@@ -54,6 +54,14 @@ class WebScrollbarLayer;
 
 using ScrollbarId = uint64_t;
 
+// ScrollingCoordinator is a page-level object that mediates interactions
+// between Blink and the compositor's scroll-related APIs on WebLayer and
+// WebScrollbarLayer.
+//
+// It's responsible for propagating scroll offsets, main-thread scrolling
+// reasons, touch action regions, and non-fast-scrollable regions into the
+// compositor, as well as creating and managing scrollbar layers.
+
 class CORE_EXPORT ScrollingCoordinator final
     : public GarbageCollectedFinalized<ScrollingCoordinator> {
   WTF_MAKE_NONCOPYABLE(ScrollingCoordinator);