Add optional debugging output of what paint chunks go into what layers.

third_party/WebKit/LayoutTests/paint/invalidation/margin-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
+      "paintChunkContents": [
+        [
+          {
+            "index": 2,
+            "type": "DrawingDocumentBackground",
+            "clientDebugName": "clientDebugName: \"LayoutView #document\"",

[chrishtr, 2016/12/15 21:26:18]

This is just an example, probably best to revert it before committing.

+            "visualRect": "[0,0 800x600]"
+          },
+          {
+            "index": 4,
+            "type": "DrawingBoxDecorationBackground",
+            "clientDebugName": "clientDebugName: \"LayoutBlockFlow (positioned) DIV id='target'\"",
+            "visualRect": "[20,20 100x100]"
+          }
+        ]
+      ],
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow (positioned) DIV id='target'",
           "rect": [20, 20, 100, 100],
           "reason": "bounds change"
         },
         {
-          "object": "LayoutBlockFlow (positioned) DIV id='target'",
+          "object": "",
           "rect": [0, 0, 100, 100],
-          "reason": "bounds change"
+          "reason": "full"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow (positioned) DIV id='target'",
       "reason": "bounds change"
     }
   ]
 }