[New Multicolumn] Disallow multicol on replaced content and fieldsets.

Source/core/layout/LayoutBlockFlow.cpp

Index: Source/core/layout/LayoutBlockFlow.cpp
diff --git a/Source/core/layout/LayoutBlockFlow.cpp b/Source/core/layout/LayoutBlockFlow.cpp
index 914c9de5a4b3720e53bc7e6ecf988c4ed8f44ce5..516ee2fefb9999aa9fce21c9145c41c9d1e9a712 100644
--- a/Source/core/layout/LayoutBlockFlow.cpp
+++ b/Source/core/layout/LayoutBlockFlow.cpp
@@ -3100,6 +3100,15 @@ void LayoutBlockFlow::createOrDestroyMultiColumnFlowThreadIfNeeded(const Compute
     if (isRuby())
         return;
 
+    // Fieldsets look for a legend special child (layoutSpecialExcludedChild()). We currently only
+    // support one special child per layout object, and the flow thread would make for a second one.
+    if (isFieldset())
+        return;
+
+    // Form controls are replaced content, and are therefore not supposed to support multicol.
+    if (isFileUploadControl() || isTextControl() || isListBox())
+        return;
+
     LayoutMultiColumnFlowThread* flowThread = createMultiColumnFlowThread(type);
     addChild(flowThread);