Add a force-layout-height-to-zero layout mode to WebViewImpl

Source/web/WebViewImpl.h

Index: Source/web/WebViewImpl.h
diff --git a/Source/web/WebViewImpl.h b/Source/web/WebViewImpl.h
index 25dcb4f8f1924629ae227b7fa939749d97bebb80..178626b87c39d5a108541b3df427cf53178ded77 100644
--- a/Source/web/WebViewImpl.h
+++ b/Source/web/WebViewImpl.h
@@ -192,6 +192,7 @@ public:
     virtual float deviceScaleFactor() const OVERRIDE;
     virtual void setDeviceScaleFactor(float) OVERRIDE;
 
+    virtual void setForceZeroLayoutHeight(bool enable) OVERRIDE;
     virtual void setFixedLayoutSize(const WebSize&) OVERRIDE;
 
     virtual void enableAutoResizeMode(
@@ -578,6 +579,7 @@ private:
 
     WebSize m_size;
     bool m_fixedLayoutSizeLock;
+    bool m_forceZeroLayoutHeight;
     // If true, automatically resize the render view around its content.
     bool m_shouldAutoResize;
     // The lower bound on the size when auto-resizing.