Enforce clearing renderers' paint invalidation state

LayoutTests/platform/linux/fast/repaint/bugzilla-6278-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [304, 138, 2, 215],
         [254, 138, 52, 235],
         [10, 323, 242, 50],
         [10, 303, 292, 50],
         [10, 138, 292, 215],
         [10, 138, 292, 160],
         [10, 138, 242, 235],
         [10, 138, 242, 180],
-        [8, 355, 784, 20],
         [8, 355, 300, 20],
-        [8, 355, 300, 20],
-        [8, 353, 300, 20],
-        [0, 363, 800, 20]
+        [8, 353, 300, 20]
       ]
     }
   ]
 }