Support 'extend-to-zoom' viewport descriptor value

Source/core/platform/Length.h

Index: Source/core/platform/Length.h
diff --git a/Source/core/platform/Length.h b/Source/core/platform/Length.h
index 6f1eb0b28013128a52908767da972025e30b52f7..3a65113b2521373434c38ad72b14fc86e53166cc 100644
--- a/Source/core/platform/Length.h
+++ b/Source/core/platform/Length.h
@@ -40,11 +40,12 @@ enum LengthType {
     MinContent, MaxContent, FillAvailable, FitContent,
     Calculated,
     ViewportPercentageWidth, ViewportPercentageHeight, ViewportPercentageMin, ViewportPercentageMax,
+    ExtendToZoom,
     Undefined
 };
 
-class CalculationValue;    
-    
+class CalculationValue;
+
 struct Length {
     WTF_MAKE_FAST_ALLOCATED;
 public:
@@ -231,6 +232,7 @@ public:
     bool isMaxContent() const { return type() == MaxContent; }
     bool isFillAvailable() const { return type() == FillAvailable; }
     bool isFitContent() const { return type() == FitContent; }
+    bool isExtendToZoom() const { return type() == ExtendToZoom; }
 
     Length blend(const Length& from, double progress) const
     {