Add a runtime flag for the column-fill property.

Source/core/layout/LayoutMultiColumnSet.cpp

Index: Source/core/layout/LayoutMultiColumnSet.cpp
diff --git a/Source/core/layout/LayoutMultiColumnSet.cpp b/Source/core/layout/LayoutMultiColumnSet.cpp
index 76f0c23fdff7d35d2ee1540b1343a08e3386c2e8..b26181c48d4fca8035b069569d4134eca5a9c760 100644
--- a/Source/core/layout/LayoutMultiColumnSet.cpp
+++ b/Source/core/layout/LayoutMultiColumnSet.cpp
@@ -30,6 +30,7 @@
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "core/layout/MultiColumnFragmentainerGroup.h"
 #include "core/paint/MultiColumnSetPainter.h"
+#include "platform/RuntimeEnabledFeatures.h"
 
 namespace blink {
 
@@ -137,8 +138,10 @@ bool LayoutMultiColumnSet::heightIsAuto() const
 {
     LayoutMultiColumnFlowThread* flowThread = multiColumnFlowThread();
     if (!flowThread->isLayoutPagedFlowThread()) {
-        if (multiColumnBlockFlow()->style()->columnFill() == ColumnFillBalance)
-            return true;
+        if (RuntimeEnabledFeatures::unprefixMulticolEnabled()) {

[Julien - ping for review, 2015/05/22 22:09:18]

Comment about why we do that and what the behavior is, por favor! The comment
from the previous iteration was great.

[mstensho (USE GERRIT), 2015/05/25 18:47:20]

Done.

+            if (multiColumnBlockFlow()->style()->columnFill() == ColumnFillBalance)
+                return true;
+        }
         if (LayoutBox* next = nextSiblingBox()) {
             if (next->isLayoutMultiColumnSpannerPlaceholder()) {
                 // If we're followed by a spanner, we need to balance.