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 32ce21f11b18dfb32d00a303b8cd633c8f848461..e15f1f74e50ddce1df106289038ceea61195d3fa 100644
--- a/Source/core/platform/Length.h
+++ b/Source/core/platform/Length.h
@@ -40,6 +40,7 @@ enum LengthType {
     MinContent, MaxContent, FillAvailable, FitContent,
     Calculated,
     ViewportPercentageWidth, ViewportPercentageHeight, ViewportPercentageMin, ViewportPercentageMax,
+    ExtendToZoom,
     Undefined
 };
 
@@ -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
     {