Add a runtime flag for the column-fill property.

Source/core/style/ComputedStyle.cpp

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 1623 matching lines @@
 }
 
 #if ENABLE(OILPAN)
 const FilterOperations& ComputedStyle::initialFilter()
 {
     DEFINE_STATIC_LOCAL(Persistent<FilterOperationsWrapper>, ops, (FilterOperationsWrapper::create()));
     return ops->operations();
 }
 #endif
 
+ColumnFill ComputedStyle::initialColumnFill()
+{
+    if (RuntimeEnabledFeatures::columnFillEnabled())

[Julien - ping for review, 2015/05/20 19:01:22]

That means that we will check this flag for every ComputedStyle we create and
introduce a branch there.

[mstensho (USE GERRIT), 2015/05/20 19:30:42]

Ouch, that's a good point. I don't wanna do it this way then.

I could fix it at a less hot spot in the layout engine instead.

+        return ColumnFillBalance;
+    // If the column-fill property isn't enabled, we want the layout engine to use column-fill:auto,
+    // so that multicol containers with specified height don't get their columns balanced
+    // (auto-height multicol containers will still get their columns balanced, even if column-fill
+    // isn't 'balance' - in accordance with the spec). This behavior also matches the old multicol
+    // implementation, which has no support for this property.

[Julien - ping for review, 2015/05/20 19:01:22]

That comment is weird. The specification seems to have said to use 'balance' as
the initial value since 2011. Is that a bug in our old implementation?

[mstensho (USE GERRIT), 2015/05/20 19:30:42]

Well, a "bug" (or rather missing feature) in the old implementation, where
column-fill wasn't supported. The old implementation balances auto-height
multicol containers (what else could you do anyway), and doesn't balance
non-auto-height multicol containers. In other words, as if column-fill were
'auto'.

[esprehn, 2015/05/20 20:26:55]

This is the wrong place to fix this, you should fix it in the getter that
getComputedStyle uses, not here.

[Julien - ping for review, 2015/05/21 14:12:47]

I don't agree, that's a super gross approach. It's a lot better to handle this
in the layout code (which I think is what Morten offered).

+    return ColumnFillAuto;
+}
+
 LayoutRectOutsets ComputedStyle::imageOutsets(const NinePieceImage& image) const
 {
     return LayoutRectOutsets(
         NinePieceImage::computeOutset(image.outset().top(), borderTopWidth()),
         NinePieceImage::computeOutset(image.outset().right(), borderRightWidth()),
         NinePieceImage::computeOutset(image.outset().bottom(), borderBottomWidth()),
         NinePieceImage::computeOutset(image.outset().left(), borderLeftWidth()));
 }
 
 void ComputedStyle::setBorderImageSource(PassRefPtr<StyleImage> image)
@@ skipping 82 matching lines @@
 }
 
 void ComputedStyle::copyChildDependentFlagsFrom(const ComputedStyle& other)
 {
     setEmptyState(other.emptyState());
     if (other.hasExplicitlyInheritedProperties())
         setHasExplicitlyInheritedProperties();
 }
 
 } // namespace blink