Bug 1014393 - Add Blob::CreateEmptyBlob. r=baku

I needed empty blobs in MediaRecorder and seeing that there was no
Blob::CreateEmptyBlob, nor an export of EmptyBlobImpl, I thought
Blob::CreateEmptyBlob would be the cleaner solution, so here it is.

Differential Revision: https://phabricator.services.mozilla.com/D35310

--HG--
extra : moz-landing-system : lando

dom/file/Blob.cpp

@@ -5,6 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "Blob.h"
+#include "EmptyBlobImpl.h"
 #include "File.h"
 #include "MemoryBlobImpl.h"
 #include "mozilla/dom/BlobBinding.h"
@@ -72,6 +73,14 @@ Blob* Blob::Create(nsISupports* aParent, BlobImpl* aImpl) {
   return aImpl->IsFile() ? new File(aParent, aImpl) : new Blob(aParent, aImpl);
 }
 
+/* static */
+already_AddRefed<Blob> Blob::CreateEmptyBlob(nsISupports* aParent,
+                                             const nsAString& aContentType) {
+  RefPtr<Blob> blob = Blob::Create(aParent, new EmptyBlobImpl(aContentType));
+  MOZ_ASSERT(!blob->mImpl->IsFile());
+  return blob.forget();
+}
+
 /* static */
 already_AddRefed<Blob> Blob::CreateStringBlob(nsISupports* aParent,
                                               const nsACString& aData,

dom/file/Blob.h

@@ -50,6 +50,9 @@ class Blob : public nsIMutable,
   // This creates a Blob or a File based on the type of BlobImpl.
   static Blob* Create(nsISupports* aParent, BlobImpl* aImpl);
 
+  static already_AddRefed<Blob> CreateEmptyBlob(nsISupports* aParent,
+                                                const nsAString& aContentType);
+
   static already_AddRefed<Blob> CreateStringBlob(nsISupports* aParent,
                                                  const nsACString& aData,
                                                  const nsAString& aContentType);