Add a runtime flag for the column-fill property.

Source/core/style/ComputedStyle.cpp

Index: Source/core/style/ComputedStyle.cpp
diff --git a/Source/core/style/ComputedStyle.cpp b/Source/core/style/ComputedStyle.cpp
index 0d17e0807421c34b9c62b6dfb2c6333563dc27c1..6967d98e2de85e264994c0ad4a2352472c5b9b2f 100644
--- a/Source/core/style/ComputedStyle.cpp
+++ b/Source/core/style/ComputedStyle.cpp
@@ -1641,6 +1641,18 @@ const FilterOperations& ComputedStyle::initialFilter()
 }
 #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(