Convert animation and renderer code to know about BorderImageLength

Source/platform/LengthOrNumber.h

Index: Source/platform/LengthOrNumber.h
diff --git a/Source/platform/LengthOrNumber.h b/Source/platform/LengthOrNumber.h
index 2d9b378aba79279bb3ad447a62263f94958f6a45..c27925e80b6dfd16091713077e636c546e3d4160 100644
--- a/Source/platform/LengthOrNumber.h
+++ b/Source/platform/LengthOrNumber.h
@@ -48,8 +48,6 @@ public:
 
     double number() const { ASSERT(isNumber()); return m_length.value(); }
 
-    Length unifiedLength() const { return m_length; }
-
     bool operator==(const LengthOrNumber& o) const
     {
         return m_length == o.m_length;