Added a console warning when <picture><source src></picture> is used.

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 e738685e6f1267e4894d29acbb622019399bce0d..f990ded90d2f06fff2f7ede63f73aa040756427b 100644
--- a/Source/core/html/HTMLImageElement.cpp
+++ b/Source/core/html/HTMLImageElement.cpp
@@ -41,6 +41,7 @@
 #include "core/html/canvas/CanvasRenderingContext.h"
 #include "core/html/parser/HTMLParserIdioms.h"
 #include "core/html/parser/HTMLSrcsetParser.h"
+#include "core/inspector/ConsoleMessage.h"
 #include "core/rendering/RenderImage.h"
 #include "platform/MIMETypeRegistry.h"
 #include "platform/RuntimeEnabledFeatures.h"
@@ -282,6 +283,8 @@ ImageCandidate HTMLImageElement::findBestFitImageFromPictureParent()
             continue;
 
         HTMLSourceElement* source = toHTMLSourceElement(child);
+        if (!source->fastGetAttribute(srcAttr).isNull())
+            document().addConsoleMessage(ConsoleMessage::create(RenderingMessageSource, WarningMessageLevel, "<source src> with a <picture> parent is invalid and therefore ignored. Use <source srcset> instead."));

[Mike West, 2014/08/13 13:57:24]

It might be worthwhile to measure how often this occurs to inform future
decisions. You could do that while writing out the message via
UseCounter::countDeprecation.

         String srcset = source->fastGetAttribute(srcsetAttr);
         if (srcset.isEmpty())
             continue;