Add a use counter to determine compat impact of changing flexbox's intrinsic size calculation

Source/core/layout/LayoutFlexibleBox.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 76 matching lines @@
     LayoutFlexibleBox* layoutObject = new LayoutFlexibleBox(nullptr);
     layoutObject->setDocumentForAnonymous(document);
     return layoutObject;
 }
 
 void LayoutFlexibleBox::computeIntrinsicLogicalWidths(LayoutUnit& minLogicalWidth, LayoutUnit& maxLogicalWidth) const
 {
     // FIXME: We're ignoring flex-basis here and we shouldn't. We can't start honoring it though until
     // the flex shorthand stops setting it to 0.
     // See https://bugs.webkit.org/show_bug.cgi?id=116117 and http://crbug.com/240765.
+    float previousMaxContentFlexFraction = -1;
     for (LayoutBox* child = firstChildBox(); child; child = child->nextSiblingBox()) {
         if (child->isOutOfFlowPositioned())
             continue;
 
         LayoutUnit margin = marginIntrinsicLogicalWidthForChild(*child);
 
         LayoutUnit minPreferredLogicalWidth;
         LayoutUnit maxPreferredLogicalWidth;
         bool hasOrthogonalWritingMode = child->isHorizontalWritingMode() != isHorizontalWritingMode();
         if (hasOrthogonalWritingMode) {
@@ skipping 11 matching lines @@
             if (isMultiline()) {
                 // For multiline, the min preferred width is if you put a break between each item.
                 minLogicalWidth = std::max(minLogicalWidth, minPreferredLogicalWidth);
             } else {
                 minLogicalWidth += minPreferredLogicalWidth;
             }
         } else {
             minLogicalWidth = std::max(minPreferredLogicalWidth, minLogicalWidth);
             maxLogicalWidth = std::max(maxPreferredLogicalWidth, maxLogicalWidth);
         }
+
+        // Determine whether the new version of the intrinsic size algorithm of the flexbox
+        // spec would produce a different result than our above algorithm.

[leviw_travelin_and_unemployed, 2015/08/20 21:45:49]

Could we maybe extract this into a function?

[cbiesinger, 2015/08/20 22:00:44]

Done

+        // The algorithm produces a different result iff the max-content flex fraction
+        // (as defined in the new algorithm) is not identical for each flex item.
+        if (isColumnFlow())
+            continue;
+        Length flexBasis = child->styleRef().flexBasis();
+        float flexGrow = child->styleRef().flexGrow();
+        // A flex-basis of auto will lead to a max-content flex fraction of zero, so just like
+        // an inflexible item it would compute to a size of max-content, so we ignore it here.
+        if (flexBasis.isAuto() || flexGrow == 0)
+            continue;
+        flexGrow = std::max(1.0f, flexGrow);
+        float maxContentFlexFraction = maxPreferredLogicalWidth.toFloat() / flexGrow;
+        if (previousMaxContentFlexFraction != -1 && maxContentFlexFraction != previousMaxContentFlexFraction)
+            UseCounter::count(document(), UseCounter::FlexboxIntrinsicSizeAlgorithmIsDifferent);
+        previousMaxContentFlexFraction = maxContentFlexFraction;
     }
 
     maxLogicalWidth = std::max(minLogicalWidth, maxLogicalWidth);
 
     LayoutUnit scrollbarWidth = intrinsicScrollbarLogicalWidth();
     maxLogicalWidth += scrollbarWidth;
     minLogicalWidth += scrollbarWidth;
 }
 
 static int synthesizedBaselineFromContentBox(const LayoutBox& box, LineDirectionMode direction)
@@ skipping 1308 matching lines @@
             ASSERT(child);
             LayoutUnit lineCrossAxisExtent = lineContexts[lineNumber].crossAxisExtent;
             LayoutUnit originalOffset = lineContexts[lineNumber].crossAxisOffset - crossAxisStartEdge;
             LayoutUnit newOffset = contentExtent - originalOffset - lineCrossAxisExtent;
             adjustAlignmentForChild(*child, newOffset - originalOffset);
         }
     }
 }
 
 }