Fix paint invalidation rect tracking within composited scrolling layers.

LayoutTests/platform/win/compositing/overflow/updating-scrolling-content-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [200, 200],
           "drawsContent": true,
           "repaintRects": [
-            [0, 0, 185, 200],
-            [0, -1000, 185, 200]
+            [0, 0, 185, 200]
           ],
           "paintInvalidationClients": [
             "LayoutBlockFlow DIV id='indicator'"
           ],
           "children": [
             {
               "bounds": [185, 185],
               "children": [
                 {
                   "bounds": [185, 1200],
                   "drawsContent": true,
                   "repaintRects": [
-                    [0, 0, 185, 200],
-                    [0, -1000, 185, 200]
+                    [0, 0, 185, 200]
                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow DIV id='indicator'"
                   ],
                   "children": [
                     {
 
                     }
                   ]
                 }
@@ skipping 24 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }