Bug 1501794 - Implement img decode API. r=bzbarsky,tnikkel

The img decode API allows a web author to request that an image be
decoded at its intrinsic size and be notified when it has been
completed. This is useful to ensure an image is ready to display before
adding it to the DOM tree -- this will help reduce flickering.

Differential Revision: https://phabricator.services.mozilla.com/D11362

dom/html/HTMLImageElement.cpp

@@ -191,6 +191,10 @@ void HTMLImageElement::GetDecoding(nsAString& aValue) {
   GetEnumAttr(nsGkAtoms::decoding, kDecodingTableDefault->tag, aValue);
 }
 
+already_AddRefed<Promise> HTMLImageElement::Decode(ErrorResult& aRv) {
+  return nsImageLoadingContent::QueueDecodeAsync(aRv);
+}
+
 bool HTMLImageElement::ParseAttribute(int32_t aNamespaceID, nsAtom* aAttribute,
                                       const nsAString& aValue,
                                       nsIPrincipal* aMaybeScriptedPrincipal,