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 9bafe5608cee455c06f8c1567d781bac88007b89..1aeab5354d21e762bb9057d3ec93c98c6a4ae2dc 100644
--- a/Source/core/loader/ImageLoader.cpp
+++ b/Source/core/loader/ImageLoader.cpp
@@ -66,8 +66,7 @@ static ImageLoader::BypassMainWorldBehavior shouldBypassMainWorldCSP(ImageLoader
 {
     ASSERT(loader);
     ASSERT(loader->element());
-    ASSERT(loader->element()->document().frame());
-    if (loader->element()->document().frame()->script().shouldBypassMainWorldCSP())
+    if (loader->element()->document().frame() && loader->element()->document().frame()->script().shouldBypassMainWorldCSP())
         return ImageLoader::BypassMainWorldCSP;
     return ImageLoader::DoNotBypassMainWorldCSP;
 }
@@ -268,10 +267,13 @@ void ImageLoader::doUpdateFromElement(BypassMainWorldBehavior bypassBehavior, Up
             crossSiteOrCSPViolationOccured(imageSourceURL);
         else
             clearFailedLoadURL();
-    } else if (!imageSourceURL.isNull()) {
-        // Fire an error event if the url string is not empty, but the KURL is.
-        m_hasPendingErrorEvent = true;
-        errorEventSender().dispatchEventSoon(this);
+    } else {
+        if (!imageSourceURL.isNull()) {
+            // Fire an error event if the url string is not empty, but the KURL is.
+            m_hasPendingErrorEvent = true;
+            errorEventSender().dispatchEventSoon(this);
+        }
+        noImageResourceToLoad();
     }
 
     ImageResource* oldImage = m_image.get();
@@ -316,7 +318,7 @@ void ImageLoader::doUpdateFromElement(BypassMainWorldBehavior bypassBehavior, Up
     updatedHasPendingEvent();
 }
 
-void ImageLoader::updateFromElement(UpdateFromElementBehavior updateBehavior, LoadType loadType)
+void ImageLoader::updateFromElement(UpdateFromElementBehavior updateBehavior)
 {
     AtomicString imageSourceURL = m_element->imageSourceURL();
 
@@ -334,7 +336,7 @@ void ImageLoader::updateFromElement(UpdateFromElementBehavior updateBehavior, Lo
     }
 
     KURL url = imageSourceToKURL(imageSourceURL);
-    if (imageSourceURL.isNull() || url.isNull() || shouldLoadImmediately(url, loadType)) {
+    if (shouldLoadImmediately(url)) {
         doUpdateFromElement(DoNotBypassMainWorldCSP, updateBehavior);
         return;
     }
@@ -358,14 +360,19 @@ KURL ImageLoader::imageSourceToKURL(AtomicString imageSourceURL) const
     return url;
 }
 
-bool ImageLoader::shouldLoadImmediately(const KURL& url, LoadType loadType) const
+bool ImageLoader::shouldLoadImmediately(const KURL& url) const
 {
+    // We force failed image loads through the asynchronous path so that we can add the shadow DOM
+    // for the alt-text content when style recalc is over and DOM mutation is allowed again.
+    if (!url.isNull()) {
+        Resource* resource = memoryCache()->resourceForURL(url);
+        if (resource && !resource->errorOccurred())
+            return true;
+    }
     return (m_loadingImageDocument
         || isHTMLObjectElement(m_element)
         || isHTMLEmbedElement(m_element)
-        || url.protocolIsData()
-        || memoryCache()->resourceForURL(url)
-        || loadType == ForceLoadImmediately);
+        || url.protocolIsData());
 }
 
 void ImageLoader::notifyFinished(Resource* resource)