Avoid over-eager clipping of child layers in multicol.

third_party/WebKit/LayoutTests/paint/invalidation/multicol-with-relpos-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow (relative positioned) DIV id='target'",
-          "rect": [508, 28, 130, 20],
+          "rect": [508, 28, 150, 20],
           "reason": "style change"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow (relative positioned) DIV id='target'",
       "reason": "style change"
     }
   ]
 }