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

Source/core/html/HTMLImageElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 23 matching lines @@
 #include "core/dom/NodeTraversal.h"
 #include "core/fetch/ImageResource.h"
 #include "core/frame/UseCounter.h"
 #include "core/html/HTMLAnchorElement.h"
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/HTMLFormElement.h"
 #include "core/html/HTMLSourceElement.h"
 #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"
 
 namespace blink {
 
 using namespace HTMLNames;
 
 class HTMLImageElement::ViewportChangeListener FINAL : public MediaQueryListListener {
 public:
@@ skipping 221 matching lines @@
     if (!parent || !isHTMLPictureElement(*parent))
         return ImageCandidate();
     for (Node* child = parent->firstChild(); child; child = child->nextSibling()) {
         if (child == this)
             return ImageCandidate();
 
         if (!isHTMLSourceElement(*child))
             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;
         String type = source->fastGetAttribute(typeAttr);
         if (!type.isEmpty() && !supportedImageType(type))
             continue;
 
         if (!source->mediaQueryMatches())
             continue;
 
@@ skipping 341 matching lines @@
     }
     imageLoader().updateFromElement(behavior);
 }
 
 const KURL& HTMLImageElement::sourceURL() const
 {
     return cachedImage()->response().url();
 }
 
 }