Update Source/core documentation references in paint and layout

Change-Id: Ifd72366f13def1a12c2f16392ff378c58b23b907
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307366
Auto-Submit: Philip Rogers <pdr@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789941}

third_party/blink/renderer/core/layout/README.md

@@ -5,7 +5,7 @@
 
 # Blink Layout
 
-The `Source/core/layout` directory contains the implementation of layout objects.
+The `renderer/core/layout` directory contains the implementation of layout objects.
 It covers the following document lifecycle states:
 
 * LayoutSubtreeChange (`InLayoutSubtreeChange` and `LayoutSubtreeChangeClean`)

third_party/blink/renderer/core/layout/layout_box_model_object.h

@@ -123,7 +123,8 @@ enum LineDirectionMode { kHorizontalLine, kVerticalLine };
 //   block coordinates", without considering text direction. Examples are
 //   "LogicalLeft" and "LogicalRight".
 //
-// For more, see Source/core/layout/README.md ### Coordinate Spaces.
+// For more information, see the following doc about coordinate spaces:
+// https://chromium.googlesource.com/chromium/src.git/+/master/third_party/blink/renderer/core/layout/README.md#coordinate-spaces
 class CORE_EXPORT LayoutBoxModelObject : public LayoutObject {
  public:
   LayoutBoxModelObject(ContainerNode*);

third_party/blink/renderer/core/page/scrolling/README.md

 # Scrolling
 
-The `Source/core/page/scrolling` directory contains utilities and classes for
+The `renderer/core/page/scrolling` directory contains utilities and classes for
 scrolling that don't belong anywhere else. For example, the majority of
 document.rootScroller's implementation as well as overscroll and some scroll
 customization types live here.

third_party/blink/renderer/core/paint/paint_invalidator.h

@@ -116,10 +116,9 @@ struct CORE_EXPORT PaintInvalidatorContext {
   const LayoutBoxModelObject* directly_composited_container = nullptr;
 
   // The current directly composited container for stacked contents (stacking
-  // contexts or positioned objects).  It is the nearest ancestor composited
-  // object which establishes a stacking context.  See
-  // Source/core/paint/README.md ### PaintInvalidationState for details on how
-  // stacked contents' paint invalidation containers differ.
+  // contexts or positioned objects). It is the nearest ancestor composited
+  // object which establishes a stacking context. For more information, see:
+  // https://chromium.googlesource.com/chromium/src.git/+/master/third_party/blink/renderer/core/paint/README.md#Stacked-elements-and-stacking-contexts
   const LayoutBoxModelObject*
       directly_composited_container_for_stacked_contents = nullptr;