Make all PaintLayers that scroll overflow self-painting.

third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/selection/selection-in-non-composited-scrolling-container-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutTextControl INPUT id='target'",
@@ skipping 22 matching lines @@
     },
     {
       "object": "RootInlineBox",
       "reason": "subtree"
     },
     {
       "object": "HorizontalScrollbar",
       "reason": "scroll control"
     },
     {
+      "object": "Caret",
+      "reason": "caret"
+    },
+    {
       "object": "LayoutText #text",
       "reason": "subtree"
     },
     {
       "object": "InlineTextBox 'test test test'",
       "reason": "subtree"
     }
   ]
 }