Use Shadow DOM to display fallback content for images

Source/core/loader/ImageLoader.cpp

Index: Source/core/loader/ImageLoader.cpp
diff --git a/Source/core/loader/ImageLoader.cpp b/Source/core/loader/ImageLoader.cpp
index a93ef263032e3ac86928e438f3d796cce8c92969..3e51b8d8ef6977f5e1c3404011295fde442c0b60 100644
--- a/Source/core/loader/ImageLoader.cpp
+++ b/Source/core/loader/ImageLoader.cpp
@@ -272,8 +272,11 @@ void ImageLoader::doUpdateFromElement(BypassMainWorldBehavior bypassBehavior, Up
         // Fire an error event if the url string is not empty, but the KURL is.
         m_hasPendingErrorEvent = true;
         errorEventSender().dispatchEventSoon(this);
+    } else {
+        noImageResourceToLoad();
     }
 
+
     ImageResource* oldImage = m_image.get();
     if (newImage != oldImage) {
         sourceImageChanged();
@@ -334,7 +337,7 @@ void ImageLoader::updateFromElement(UpdateFromElementBehavior updateBehavior, Lo
     }
 
     KURL url = imageSourceToKURL(imageSourceURL);
-    if (imageSourceURL.isNull() || url.isNull() || shouldLoadImmediately(url, loadType)) {
+    if ((!imageSourceURL.isNull() && url.isNull()) || shouldLoadImmediately(url, loadType)) {
         doUpdateFromElement(DoNotBypassMainWorldCSP, updateBehavior);
         return;
     }
@@ -364,7 +367,8 @@ bool ImageLoader::shouldLoadImmediately(const KURL& url, LoadType loadType) cons
         || isHTMLObjectElement(m_element)
         || isHTMLEmbedElement(m_element)
         || url.protocolIsData()
-        || memoryCache()->resourceForURL(url)
+        || !element()->document().frame()
+        || (!url.isNull() && memoryCache()->resourceForURL(url))
         || loadType == ForceLoadImmediately);
 }