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

Source/web/PageScaleConstraintsSet.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 15 matching lines @@
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef PageScaleConstraintsSet_h
 #define PageScaleConstraintsSet_h
 
 #include "core/dom/ViewportDescription.h"
 #include "core/page/PageScaleConstraints.h"
+#include "platform/Length.h"
 #include "platform/geometry/IntSize.h"
 
 namespace blink {
 
 // This class harmonizes the viewport (particularly page scale) constraints from
 // the meta viewport tag and other sources.
 class PageScaleConstraintsSet {
 public:
     PageScaleConstraintsSet();
 
     WebCore::PageScaleConstraints defaultConstraints() const;
 
     // 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
     // page-defined values.
     const WebCore::PageScaleConstraints& userAgentConstraints() const { return m_userAgentConstraints; }
     void setUserAgentConstraints(const WebCore::PageScaleConstraints&);
 
     // Actual computed values, taking into account the above plus the current
     // viewport size and document width.
     const WebCore::PageScaleConstraints& finalConstraints() const { return m_finalConstraints; }
@@ skipping 19 matching lines @@
 
     int m_lastContentsWidth;
 
     bool m_needsReset;
     bool m_constraintsDirty;
 };
 
 } // namespace WebCore
 
 #endif // PageScaleConstraintsSet_h