Disentangle repaint-after-compositing from updateLayerPositionsAfterLayout

LayoutTests/platform/mac/fast/repaint/float-overflow-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [67, 426, 50, 10],
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+        [63, 21, 54, 18],
+        [63, 21, 42, 18],
+        [63, 15, 54, 18],
+        [63, 15, 42, 18],

[Julien - ping for review, 2014/08/01 23:50:05]

This is my biggest concern with this patch. The code is fine and I have no
objection to it.

While I don't think the extra invalidations are costing us a lot, it's still not
correct to generate tons of extra redundant invalidations. We have tried to
reduce them as we implemented repaint-after-layout and it's a clear regression
from that perspective.

If the patch was solving a correctness bug, a case could be made for the extra
invalidations. However this change is a refactoring so it's a pure regression
from my perspective.

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     }
   ]
 }