Enable srcset support in HTMLImageElement

Source/core/html/HTMLImageElement.cpp

Index: Source/core/html/HTMLImageElement.cpp
diff --git a/Source/core/html/HTMLImageElement.cpp b/Source/core/html/HTMLImageElement.cpp
index bf88109ed9ecad7b640245f02d5216edaa77a176..6d82685bff8ff07cb43ddfbe62d3c047eb49e375 100644
--- a/Source/core/html/HTMLImageElement.cpp
+++ b/Source/core/html/HTMLImageElement.cpp
@@ -25,6 +25,7 @@
 
 #include "CSSPropertyNames.h"
 #include "HTMLNames.h"
+#include "RuntimeEnabledFeatures.h"
 #include "bindings/v8/ScriptEventListener.h"
 #include "core/dom/Attribute.h"
 #include "core/dom/EventNames.h"
@@ -107,13 +108,21 @@ void HTMLImageElement::collectStyleForPresentationAttribute(const QualifiedName&
         HTMLElement::collectStyleForPresentationAttribute(name, value, style);
 }
 
+const AtomicString HTMLImageElement::imageSourceURL() const
+{
+    return m_bestFitImageURL.isEmpty() ? fastGetAttribute(srcAttr) : m_bestFitImageURL;

[do-not-use, 2013/09/12 08:23:53]

Wouldn't isNull() suffice? Can bestFitSourceForImageAttributes() really return
an empty string?

[Yoav Weiss, 2013/09/12 09:30:20]

m_bestFitImageURL is initialized as an empty string, and
bestFitSourceImageAttributes() can return an empty string, if both srcSttr and
srcsetAttr's values are empty (or srcsetAttr's value is invalid).

Both can be modified to a null string, which will enable an isNull check here.
What are the benefits? Is isNull() faster than isEmpty()?

[do-not-use, 2013/09/12 09:56:37]

To be clear the default constructor for String creates a NULL string, not an
empty one. Unless I am mistaken, m_bestFitImageURL is initially a NULL string
and bestFitSourceImageAttributes() may return String(), which is a NULL string
as well.

To answer your question, yes, isNull() is faster than isEmpty() still isEmpty()
checks if the string is null or empty (2 checks instead of 1).

+}
+
 void HTMLImageElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
     if (name == altAttr) {
         if (renderer() && renderer()->isImage())
             toRenderImage(renderer())->updateAltText();
-    } else if (name == srcAttr)
+    } else if (name == srcAttr || name == srcsetAttr) {
+        if (RuntimeEnabledFeatures::srcsetEnabled())
+            m_bestFitImageURL = bestFitSourceForImageAttributes(document().devicePixelRatio(), fastGetAttribute(srcAttr), fastGetAttribute(srcsetAttr));
         m_imageLoader.updateFromElementIgnoringPreviousError();
+    }
     else if (name == usemapAttr)
         setIsLink(!value.isNull());
     else if (name == onbeforeloadAttr)