Issue #1442 - Part 10a - Unify body extraction in Fetch/Beacon/XHR. https://bugzilla.mozilla.org/show_bug.cgi?id=1329298 Pre-requisite for Part 11.

dom/fetch/BodyExtractor.h

@@ -0,0 +1,46 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_BodyExtractor_h
+#define mozilla_dom_BodyExtractor_h
+
+#include "nsString.h"
+
+class nsIInputStream;
+
+namespace mozilla {
+namespace dom {
+
+class BodyExtractorBase
+{
+public:
+  virtual nsresult GetAsStream(nsIInputStream** aResult,
+                               uint64_t* aContentLength,
+                               nsACString& aContentTypeWithCharset,
+                               nsACString& aCharset) const = 0;
+};
+
+// The implementation versions of this template are:
+// ArrayBuffer, ArrayBufferView, Blob, FormData, nsAString, nsIDocument,
+// nsIInputStream, nsIXHRSendable, URLSearchParams
+template<typename Type>
+class BodyExtractor final : public BodyExtractorBase
+{
+  Type* mBody;
+public:
+  explicit BodyExtractor(Type* aBody) : mBody(aBody)
+  {}
+
+  nsresult GetAsStream(nsIInputStream** aResult,
+                       uint64_t* aContentLength,
+                       nsACString& aContentTypeWithCharset,
+                       nsACString& aCharset) const override;
+};
+
+} // dom namespace
+} // mozilla namespace
+
+#endif // mozilla_dom_BodyExtractor_h