Use css-device-adapt constraining for legacy viewport tags.

Source/core/css/StyleRule.cpp

Index: Source/core/css/StyleRule.cpp
diff --git a/Source/core/css/StyleRule.cpp b/Source/core/css/StyleRule.cpp
index 4e375459eeff673203a175ee1fe3ed87d61842bd..2fc1fdafbc3d982b658b041cee8d11864391cad8 100644
--- a/Source/core/css/StyleRule.cpp
+++ b/Source/core/css/StyleRule.cpp
@@ -342,14 +342,12 @@ StyleRuleRegion::StyleRuleRegion(const StyleRuleRegion& o)
 StyleRuleViewport::StyleRuleViewport()
     : StyleRuleBase(Viewport)
 {
-    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled());
 }
 
 StyleRuleViewport::StyleRuleViewport(const StyleRuleViewport& o)
     : StyleRuleBase(o)
     , m_properties(o.m_properties->mutableCopy())
 {
-    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled());
 }
 
 StyleRuleViewport::~StyleRuleViewport()