Read the default min-width value from the UA style sheets

Source/web/PageScaleConstraintsSet.h

Index: Source/web/PageScaleConstraintsSet.h
diff --git a/Source/web/PageScaleConstraintsSet.h b/Source/web/PageScaleConstraintsSet.h
index b31d4e4b1255ac4520697eb237b3a31a704f921e..af141883abfc295018d2d60ee19d0cba2f458245 100644
--- a/Source/web/PageScaleConstraintsSet.h
+++ b/Source/web/PageScaleConstraintsSet.h
@@ -33,6 +33,7 @@
 
 #include "core/dom/ViewportDescription.h"
 #include "core/page/PageScaleConstraints.h"
+#include "platform/Length.h"
 #include "platform/geometry/IntSize.h"
 
 namespace blink {
@@ -48,7 +49,7 @@ public:
     // Settings defined in the website's viewport tag, if viewport tag support
     // is enabled.
     const WebCore::PageScaleConstraints& pageDefinedConstraints() const { return m_pageDefinedConstraints; }
-    void updatePageDefinedConstraints(const WebCore::ViewportDescription&, WebCore::IntSize viewSize);
+    void updatePageDefinedConstraints(const WebCore::ViewportDescription&, WebCore::IntSize viewSize, WebCore::Length legacyFallbackWidth);
     void adjustForAndroidWebViewQuirks(const WebCore::ViewportDescription&, WebCore::IntSize viewSize, int layoutFallbackWidth, float deviceScaleFactor, bool supportTargetDensityDPI, bool wideViewportQuirkEnabled, bool useWideViewport, bool loadWithOverviewMode, bool nonUserScalableQuirkEnabled);
 
     // Constraints may also be set from Chromium -- this overrides any