A sizes attribute parser

Source/core/css/MediaValuesDynamic.h

Index: Source/core/css/MediaValuesDynamic.h
diff --git a/Source/core/css/MediaValuesDynamic.h b/Source/core/css/MediaValuesDynamic.h
new file mode 100644
index 0000000000000000000000000000000000000000..d10fe9a0337f6358e190b962bc30ac3ed2d57ffe
--- /dev/null
+++ b/Source/core/css/MediaValuesDynamic.h
@@ -0,0 +1,47 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MediaValuesDynamic_h
+#define MediaValuesDynamic_h
+
+#include "core/css/MediaValues.h"
+
+namespace WebCore {
+
+class Document;
+
+class MediaValuesDynamic FINAL : public MediaValues {
+public:
+    static PassRefPtr<MediaValues> create(Document&);
+    static PassRefPtr<MediaValues> create(PassRefPtr<LocalFrame>, PassRefPtr<RenderStyle>);
+    virtual PassRefPtr<MediaValues> copy() const OVERRIDE;
+    virtual bool isSafeToSendToAnotherThread() const OVERRIDE;
+    virtual bool computeLength(double value, unsigned short type, int& result) const OVERRIDE;
+
+    virtual int viewportWidth() const OVERRIDE;
+    virtual int viewportHeight() const OVERRIDE;
+    virtual int deviceWidth() const OVERRIDE;
+    virtual int deviceHeight() const OVERRIDE;
+    virtual float devicePixelRatio() const OVERRIDE;
+    virtual int colorBitsPerComponent() const OVERRIDE;
+    virtual int monochromeBitsPerComponent() const OVERRIDE;
+    virtual PointerDeviceType pointer() const OVERRIDE;
+    virtual bool threeDEnabled() const OVERRIDE;
+    virtual bool scanMediaType() const OVERRIDE;
+    virtual bool screenMediaType() const OVERRIDE;
+    virtual bool printMediaType() const OVERRIDE;
+    virtual bool strictMode() const OVERRIDE;
+    virtual Document* document() const OVERRIDE;
+    virtual bool hasValues() const OVERRIDE;
+
+protected:
+    MediaValuesDynamic(PassRefPtr<LocalFrame>, PassRefPtr<RenderStyle>);
+
+    RefPtr<RenderStyle> m_style;

[eseidel, 2014/04/15 18:47:03]

I suspect we could have answered Elliot's concern with a slightly simpler
design.

MediaValues would have two OwnPtrs.  One which points to MediaValuesCached, and
the other which points to MediaValuesDyanmic.  It can never have both at the
same time (RELEASE_ASSERT) and MediaValuesDynamic has
RELEASE_ASSERT(isMainThread) in its' constructor.  Then all your accessors look
like this:

if (m_dynamic)
   reutrn m_dynamic->foo();
return m_cached->foo();

All the calculate logic, etc. would just live on MediaValuesDynamic, etc.

But I think this design, making the virtual dispatch explicit is also fine. 
This is what virtual dispatch is for after all. :)

+    RefPtr<LocalFrame> m_frame;
+};
+
+} // namespace
+
+#endif // MediaValuesDynamic_h