Add preloader support for picture based source selection

Source/core/html/parser/HTMLPreloadScanner.h

Index: Source/core/html/parser/HTMLPreloadScanner.h
diff --git a/Source/core/html/parser/HTMLPreloadScanner.h b/Source/core/html/parser/HTMLPreloadScanner.h
index 8117458c98585c4aa48e371e5603d1393ee35c91..8ce44cef8a6dd966ca8d99de8735a248b07a8a1f 100644
--- a/Source/core/html/parser/HTMLPreloadScanner.h
+++ b/Source/core/html/parser/HTMLPreloadScanner.h
@@ -74,15 +74,17 @@ private:
     void updatePredictedBaseURL(const Token&);
 
     struct Checkpoint {
-        Checkpoint(const KURL& predictedBaseElementURL, bool inStyle, size_t templateCount)
+        Checkpoint(const KURL& predictedBaseElementURL, bool inStyle, const Vector<String>& pictureSourceURLStack, size_t templateCount)
             : predictedBaseElementURL(predictedBaseElementURL)
             , inStyle(inStyle)
+            , pictureSourceURLStack(pictureSourceURLStack)

[eseidel, 2014/05/09 07:30:36]

Why do these need to be stored on the checkpoint?  I guess this is part of the
fake-tree-builder state?

             , templateCount(templateCount)
         {
         }
 
         KURL predictedBaseElementURL;
         bool inStyle;
+        Vector<String> pictureSourceURLStack;
         size_t templateCount;
     };
 
@@ -90,6 +92,7 @@ private:
     const KURL m_documentURL;
     KURL m_predictedBaseElementURL;
     bool m_inStyle;
+    Vector<String> m_pictureSourceURLStack;
     size_t m_templateCount;
     RefPtr<MediaValues> m_mediaValues;