Use Shadow DOM to display fallback content for images

Source/core/html/HTMLImageLoader.cpp

Index: Source/core/html/HTMLImageLoader.cpp
diff --git a/Source/core/html/HTMLImageLoader.cpp b/Source/core/html/HTMLImageLoader.cpp
index 44fc879fe2f85abc3b3d5fe90ff5fef691766d39..3af5488be4aab50951e69409816efc90cf9d6df9 100644
--- a/Source/core/html/HTMLImageLoader.cpp
+++ b/Source/core/html/HTMLImageLoader.cpp
@@ -24,15 +24,38 @@
 
 #include "core/HTMLNames.h"
 #include "core/dom/Element.h"
+#include "core/dom/Microtask.h"
 #include "core/events/Event.h"
 #include "core/fetch/ImageResource.h"
-#include "core/html/HTMLImageElement.h"
 #include "core/html/HTMLObjectElement.h"
 #include "core/html/parser/HTMLParserIdioms.h"
 #include "platform/Logging.h"
+#include "public/platform/WebThread.h"
 
 namespace blink {
 
+using namespace HTMLNames;
+
+class HTMLImageLoader::EnsureFallbackContentTask : public blink::WebThread::Task {
+public:
+    static PassOwnPtr<EnsureFallbackContentTask> create(HTMLElement* element)
+    {
+        return adoptPtr(new EnsureFallbackContentTask(element));
+    }
+
+    EnsureFallbackContentTask(HTMLElement* element)
+        : m_element(element)
+    {
+    }
+    virtual void run() override
+    {
+        m_element->ensureFallbackContent();
+    }
+
+private:
+    HTMLElement* m_element;
+};
+
 HTMLImageLoader::HTMLImageLoader(Element* node)
     : ImageLoader(node)
 {
@@ -61,17 +84,36 @@ String HTMLImageLoader::sourceURI(const AtomicString& attr) const
     return stripLeadingAndTrailingHTMLSpaces(attr);
 }
 
+void HTMLImageLoader::noImageResourceToLoad()
+{
+    // FIXME: Use fallback content even when there is no alt-text. The only blocker is the large amount of rebaselining it requires.
+    if (!toHTMLElement(element())->altText().isEmpty())
+        toHTMLElement(element())->ensureFallbackContent();
+}
+
 void HTMLImageLoader::notifyFinished(Resource*)
 {
     ImageResource* cachedImage = image();
 
-    RefPtrWillBeRawPtr<Element> element = this->element();
     ImageLoader::notifyFinished(cachedImage);
 
     bool loadError = cachedImage->errorOccurred() || cachedImage->response().httpStatusCode() >= 400;
+    if (loadError)
+        ensureFallbackContent();
+    else
+        toHTMLElement(element())->ensurePrimaryContent();
 
-    if (loadError && isHTMLObjectElement(*element))
-        toHTMLObjectElement(element)->renderFallbackContent();
+    if (loadError && isHTMLObjectElement(element()))
+        toHTMLObjectElement(element())->renderFallbackContent();
 }
 
+void HTMLImageLoader::ensureFallbackContent()
+{
+    if (image()->url().protocolIsData()) {
+        OwnPtr<EnsureFallbackContentTask> task = EnsureFallbackContentTask::create(toHTMLElement(element()));
+        Microtask::enqueueMicrotask(task.release());
+    } else {
+        toHTMLElement(element())->ensureFallbackContent();
+    }
+}
 }