Implement left property animation on InvalidatableStyleInterpolation

Source/core/style/ComputedStyle.h

Index: Source/core/style/ComputedStyle.h
diff --git a/Source/core/style/ComputedStyle.h b/Source/core/style/ComputedStyle.h
index 3ad70e15ded5c112024236f07aa6924360329742..80ef10cbfa2f703a809a34e32d5401037d36f004 100644
--- a/Source/core/style/ComputedStyle.h
+++ b/Source/core/style/ComputedStyle.h
@@ -133,6 +133,7 @@ class CORE_EXPORT ComputedStyle: public RefCounted<ComputedStyle> {
     friend class ComputedStyleCSSValueMapping; // Needs to be able to see visited and unvisited colors for devtools.
     friend class StyleBuilderFunctions; // Sets color styles
     friend class CachedUAStyle; // Saves Border/Background information for later comparison.
+    friend class LengthPropertyFunctions; // Reads initial style values.
 
     // FIXME: When we stop resolving currentColor at style time, these can be removed.
     friend class CSSToStyleMap;