Translate viewport related meta tags into @viewport descriptors as suggested by the CSS Device Adap…

Source/core/css/html.css

Index: Source/core/css/html.css
diff --git a/Source/core/css/html.css b/Source/core/css/html.css
index e9f2b6d514483ed763f62f13b4c0ea8dc865d786..e7ae7165ffd0f39b10f0f5b6c3e9284498f11ddb 100644
--- a/Source/core/css/html.css
+++ b/Source/core/css/html.css
@@ -1118,5 +1118,14 @@ bdo {
     border-width: 0px;
 }
 
+/* viewport */
+
+@viewport {
+    -internal-priority: 0;
+    width: -internal-extend-to-zoom 980px;
+    min-zoom: 0.25;
+    max-zoom: 5;
+}
+
 /* noscript is handled internally, as it depends on settings. */