Fix paint invalidation rect tracking within composited scrolling layers.

LayoutTests/platform/win/fast/repaint/overflow-scroll-after-move-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [10, 60],
           "bounds": [300, 400],
           "drawsContent": true,
           "repaintRects": [
             [50, 160, 200, 50],
-            [50, 50, 200, 50]
+            [50, -100, 200, 50]

[Xianzhu, 2015/08/13 21:24:42]

FYI This is because crbug.com/416535, which I didn't find a good solution
without slimming paint.

           ],
           "paintInvalidationClients": [
             "LayoutBlockFlow (positioned) DIV id='block'"
           ],
           "children": [
             {
               "bounds": [285, 385],
               "children": [
                 {
                   "bounds": [285, 900],
                   "drawsContent": true,
                   "repaintRects": [
                     [50, 310, 200, 50],
-                    [50, 200, 200, 50]
+                    [50, 50, 200, 50]

[Xianzhu, 2015/08/13 21:24:42]

This seems incorrect, too.

[chrishtr, 2015/08/13 21:44:27]

Yep. Same issue it seems: absolute positioning.

                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow (positioned) DIV id='block'"
                   ],
                   "children": [
                     {
 
                     }
                   ]
                 }
@@ skipping 24 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }