[NotLanded] Implement the fetching algorithm of the Web Manifest.

public/platform/WebManifest.h

Index: public/platform/WebManifest.h
diff --git a/Source/core/html/HTMLPictureElement.idl b/public/platform/WebManifest.h
similarity index 59%
copy from Source/core/html/HTMLPictureElement.idl
copy to public/platform/WebManifest.h
index 649316cb8a1be19e29c17f14bfbc5b6e10397164..68b7b70bdc797e694c4ecc8869321bbee9b98be5 100644
--- a/Source/core/html/HTMLPictureElement.idl
+++ b/public/platform/WebManifest.h
@@ -2,9 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// http://picture.responsiveimages.org
-[
-    RuntimeEnabled=Picture
-] interface HTMLPictureElement : HTMLElement {
+#ifndef WebManifest_h
+#define WebManifest_h
+
+namespace blink {
+
+struct WebManifest {
 };
 
+} // namespace blink
+
+#endif