Move the CSS Device Adaptation @viewport rule support behind a runtime flag

Source/core/css/CSSGrammar.y.in

Index: Source/core/css/CSSGrammar.y.in
diff --git a/Source/core/css/CSSGrammar.y.in b/Source/core/css/CSSGrammar.y.in
index 9bc168f7d4e720b669d4fc3a23690095b686e2f7..a78ab42d77d85521a98b90ae35ccfb92157a7be2 100644
--- a/Source/core/css/CSSGrammar.y.in
+++ b/Source/core/css/CSSGrammar.y.in
@@ -232,9 +232,7 @@ static inline bool isCSSTokenAString(int yytype)
 %type <rule> block_valid_rule
 %type <rule> region
 %type <rule> supports
-#if ENABLE_CSS_DEVICE_ADAPTATION
 %type <rule> viewport
-#endif
 %type <rule> filter
 
 %type <string> maybe_ns_prefix
@@ -440,9 +438,7 @@ valid_rule:
   | region
   | supports
   | host
-#if ENABLE_CSS_DEVICE_ADAPTATION
   | viewport
-#endif
   | filter
   ;
 
@@ -492,9 +488,7 @@ block_valid_rule:
   | media
   | keyframes
   | supports
-#if ENABLE_CSS_DEVICE_ADAPTATION
   | viewport
-#endif
   | filter
   ;
 
@@ -1018,7 +1012,6 @@ host:
     }
     ;
 
-#if ENABLE_CSS_DEVICE_ADAPTATION
 before_viewport_rule:
     /* empty */ {
         parser->markViewportRuleBodyStart();
@@ -1038,7 +1031,6 @@ viewport:
         parser->markViewportRuleBodyEnd();
     }
 ;
-#endif
 
 region_selector:
     selector_list {