Fix paint invalidation rect tracking within composited scrolling layers.

LayoutTests/platform/linux/compositing/repaint/should-not-clip-composited-overflow-scrolling-layer-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [500, 500],
           "drawsContent": true,
           "repaintRects": [
-            [0, 20, 821, 19],
-            [0, 0, 5000, 5000],
-            [0, 0, 801, 19],
-            [-2000, -1980, 821, 19],
-            [-2000, -2000, 5000, 5000],
-            [-2000, -2000, 801, 19]
+            [-2000, -2000, 5000, 5000]
           ],
           "paintInvalidationClients": [
-            "LayoutBlockFlow DIV id='content'",
-            "LayoutText #text",
-            "InlineTextBox 'Tests invalidation of scrolling layer. Passes if the repaint rect is not clipped,\n'",
-            "InlineTextBox 'and there is no red when the scrolling container is scrolled.'",
-            "LayoutBR BR",
-            "InlineTextBox '\n'",
-            "LayoutText #text",
-            "InlineTextBox 'Note for manual testing: must run with --enable-prefer-compositing-to-lcd-text\n'",
-            "InlineTextBox 'on non-high-dpi machines to enable composited scrolling.'"
+            "LayoutBlockFlow DIV id='content'"
           ],
           "children": [
             {
               "bounds": [485, 485],
               "children": [
                 {
                   "bounds": [5000, 5000],
                   "drawsContent": true,
                   "repaintRects": [
-                    [2000, 2020, 821, 19],
-                    [2000, 2000, 5000, 5000],
-                    [2000, 2000, 801, 19],
-                    [0, 20, 821, 19],
-                    [0, 0, 5000, 5000],
-                    [0, 0, 801, 19]
+                    [0, 0, 5000, 5000]
                   ],
                   "paintInvalidationClients": [
-                    "LayoutBlockFlow DIV id='content'",
-                    "LayoutText #text",
-                    "InlineTextBox 'Tests invalidation of scrolling layer. Passes if the repaint rect is not clipped,\n'",
-                    "InlineTextBox 'and there is no red when the scrolling container is scrolled.'",
-                    "LayoutBR BR",
-                    "InlineTextBox '\n'",
-                    "LayoutText #text",
-                    "InlineTextBox 'Note for manual testing: must run with --enable-prefer-compositing-to-lcd-text\n'",
-                    "InlineTextBox 'on non-high-dpi machines to enable composited scrolling.'"
+                    "LayoutBlockFlow DIV id='content'"
                   ],
                   "children": [
                     {
 
                     }
                   ]
                 }
               ]
             },
             {
@@ skipping 27 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }