Don't invalidate width for children unless scrollbars have changed.

third_party/WebKit/LayoutTests/css3/flexbox/scrollbars-changed-expected.txt

 {
   "name": "Content Root Layer",
   "bounds": [800, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
-          "object": "LayoutBlockFlow DIV id='target'",
-          "rect": [22, 50, 226, 166],
+          "object": "LayoutBlockFlow DIV id='dynamic' class='content'",
+          "rect": [0, 0, 185, 20],
           "reason": "invalidate paint rectangle"
         },
         {
-          "object": "LayoutBlockFlow DIV id='target'",
-          "rect": [22, 50, 226, 166],
-          "reason": "invalidate paint rectangle"
+          "object": "LayoutBlockFlow DIV id='scroller'",
+          "rect": [185, 0, 15, 100],
+          "reason": "scroll"
         },
         {
           "object": "LayoutText #text",
-          "rect": [23, 51, 71, 109],
+          "rect": [0, 5, 15, 15],
           "reason": "full"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "InlineTextBox ''",
       "reason": "full"
     },
     {
       "object": "RootInlineBox",
       "reason": "full"
     },
     {
-      "object": "LayoutBlockFlow DIV id='target'",
+      "object": "VerticalScrollbar",
+      "reason": "scroll"
+    },
+    {
+      "object": "LayoutBlockFlow DIV id='dynamic' class='content'",
       "reason": "invalidate paint rectangle"
     },
     {
-      "object": "LayoutBlockFlow DIV id='target'",
-      "reason": "invalidate paint rectangle"
-    },
-    {
       "object": "LayoutText #text",
       "reason": "full"
     },
     {
-      "object": "InlineTextBox 'PASS'",
+      "object": "InlineTextBox 'z'",
       "reason": "full"
     }
   ]
 }