Support 'extend-to-zoom' viewport descriptor value

Source/core/css/LengthFunctions.cpp

Index: Source/core/css/LengthFunctions.cpp
diff --git a/Source/core/css/LengthFunctions.cpp b/Source/core/css/LengthFunctions.cpp
index 2f76efdd90268691c80cb60438b88e43b1fb4e3e..28c98deb6a0d5de3cc77dc8e05820f6097ddb4c4 100644
--- a/Source/core/css/LengthFunctions.cpp
+++ b/Source/core/css/LengthFunctions.cpp
@@ -81,6 +81,7 @@ LayoutUnit minimumValueForLength(const Length& length, LayoutUnit maximumValue,
     case MinContent:
     case MaxContent:
     case FitContent:
+    case ExtendToZoom:
     case Undefined:
         ASSERT_NOT_REACHED();
         return 0;
@@ -109,6 +110,7 @@ LayoutUnit valueForLength(const Length& length, LayoutUnit maximumValue, RenderV
     case MinContent:
     case MaxContent:
     case FitContent:
+    case ExtendToZoom:
     case Undefined:
         ASSERT_NOT_REACHED();
         return 0;
@@ -155,6 +157,7 @@ float floatValueForLength(const Length& length, float maximumValue, RenderView*
     case MinContent:
     case MaxContent:
     case FitContent:
+    case ExtendToZoom:
     case Undefined:
         ASSERT_NOT_REACHED();
         return 0;