Fix paint invalidation rect tracking within composited scrolling layers.

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

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [10, 60],
           "bounds": [700, 400],
           "drawsContent": true,
           "repaintRects": [
-            [300, 100, 120, 50],
+            [300, 0, 120, 50],
             [50, 100, 120, 50]
           ],
           "paintInvalidationClients": [
             "LayoutBlockFlow (positioned) DIV",
             "LayoutBlockFlow (positioned) DIV id='block'"
           ],
           "children": [
             {
               "bounds": [685, 385],
               "children": [
                 {
                   "bounds": [685, 600],
                   "drawsContent": true,
                   "repaintRects": [
-                    [300, 200, 120, 50],
+                    [300, 100, 120, 50],

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

This looks incorrect. The old location of the block should be at 300,200,120,50
in the scrolling contents layer.

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

Yep you're right. If the child is absolutely positioned, including scroll offset
is wrong somehow as I've written it. Investigating.

                     [50, 200, 120, 50]
                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow (positioned) DIV",
                     "LayoutBlockFlow (positioned) DIV id='block'"
                   ],
                   "children": [
                     {
 
                     }
@@ skipping 26 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }