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

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index b5fc9e6759344aaab4c94996e5abf9f17d07b9cc..f73327b7e2d28a0bc3d01ab2087e5bf06aa46f44 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -289,8 +289,11 @@ public:
     DEFINE_ATTRIBUTE_EVENT_LISTENER(webkitvisibilitychange);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(securitypolicyviolation);
 
-    void setViewportArguments(const ViewportArguments& viewportArguments) { m_viewportArguments = viewportArguments; }
+    void setViewportArguments(const ViewportArguments&);
     const ViewportArguments& viewportArguments() const { return m_viewportArguments; }
+    // When no author style for @viewport is present, and a meta tag for defining the
+    // viewport is present, apply the meta tag viewport arguments instead of the UA styles.
+    bool useLegacyViewportArguments();
 #ifndef NDEBUG
     bool didDispatchViewportPropertiesChanged() const { return m_didDispatchViewportPropertiesChanged; }
 #endif
@@ -417,6 +420,7 @@ public:
     bool isFrameSet() const;
 
     bool isSrcdocDocument() const { return m_isSrcdocDocument; }
+    bool isMobileDocument() const { return m_isMobileDocument; }
 
     StyleResolver* styleResolverIfExists() const { return m_styleResolver.get(); }
 
@@ -1308,6 +1312,7 @@ private:
     bool m_isViewSource;
     bool m_sawElementsInKnownNamespaces;
     bool m_isSrcdocDocument;
+    bool m_isMobileDocument;
 
     RenderObject* m_renderer;
     RefPtr<DocumentEventQueue> m_eventQueue;
@@ -1324,6 +1329,7 @@ private:
     Timer<Document> m_loadEventDelayTimer;
 
     ViewportArguments m_viewportArguments;
+    ViewportArguments m_legacyViewportArguments;
 
     ReferrerPolicy m_referrerPolicy;
 
@@ -1397,6 +1403,21 @@ inline const Document* Document::templateDocument() const
     return m_templateDocument.get();
 }
 
+inline void Document::setViewportArguments(const ViewportArguments& viewportArguments)
+{
+    m_viewportArguments = viewportArguments;
+    updateViewportArguments();
+}
+
+inline bool Document::useLegacyViewportArguments()
+{
+    if (m_legacyViewportArguments.type == ViewportArguments::UserAgentStyleSheet)
+        return false;
+
+    setViewportArguments(m_legacyViewportArguments);
+    return true;
+}
+
 inline Document* toDocument(ScriptExecutionContext* scriptExecutionContext)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!scriptExecutionContext || scriptExecutionContext->isDocument());