Factor out common BorderImageLength code in CSSToStyleMap

Source/core/rendering/style/BorderImageLength.h

Index: Source/core/rendering/style/BorderImageLength.h
diff --git a/Source/core/rendering/style/BorderImageLength.h b/Source/core/rendering/style/BorderImageLength.h
index 7398760876de64ba95124af9f58493e7d48f741d..a3a954feeb1770792a9f3d7c737c1ef111bdddfe 100644
--- a/Source/core/rendering/style/BorderImageLength.h
+++ b/Source/core/rendering/style/BorderImageLength.h
@@ -41,13 +41,6 @@ namespace WebCore {
 // http://www.w3.org/TR/css3-background/#border-image-outset
 class BorderImageLength {
 public:
-    BorderImageLength()
-        : m_length(Auto)
-        , m_number(0)
-        , m_type(LengthType)
-    {
-    }
-
     BorderImageLength(double number)
         : m_length(Undefined)
         , m_number(number)