Support subpixel layout for borders.

third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/repaint-during-scroll-with-zoom-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#C0C0C0",
       "paintInvalidations": [
         {
           "object": "LayoutIFrame (positioned) IFRAME",
-          "rect": [2, 64, 235, 236],
+          "rect": [2, 65, 236, 235],
           "reason": "invalidate paint rectangle"
         },
         {
           "object": "LayoutBlockFlow BODY",
-          "rect": [2, 65, 235, 235],
+          "rect": [3, 65, 235, 235],
           "reason": "subtree"
         },
         {
           "object": "LayoutView #document",
-          "rect": [2, 65, 235, 235],
+          "rect": [3, 65, 235, 235],
           "reason": "subtree"
         },
         {
           "object": "LayoutView #document",
-          "rect": [2, 300, 235, 15],
+          "rect": [3, 300, 235, 15],
           "reason": "scroll"
         },
         {
           "object": "LayoutBlockFlow HTML",
-          "rect": [2, 65, 225, 235],
+          "rect": [3, 65, 225, 235],
           "reason": "subtree"
         },
         {
           "object": "LayoutText #text",
-          "rect": [2, 65, 58, 16],
+          "rect": [3, 65, 58, 16],
           "reason": "subtree"
         },
         {
           "object": "LayoutView #document",
-          "rect": [237, 65, 15, 235],
+          "rect": [238, 65, 15, 235],
           "reason": "scroll"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutIFrame (positioned) IFRAME",
       "reason": "invalidate paint rectangle"
     },
@@ skipping 25 matching lines @@
       "object": "LayoutText #text",
       "reason": "subtree"
     },
     {
       "object": "InlineTextBox 'scroll me'",
       "reason": "subtree"
     }
   ]
 }