Introduce CompositeDataConsumerHandle.

Source/modules/fetch/CompositeDataConsumerHandle.h

Index: Source/modules/fetch/CompositeDataConsumerHandle.h
diff --git a/Source/modules/fetch/CompositeDataConsumerHandle.h b/Source/modules/fetch/CompositeDataConsumerHandle.h
new file mode 100644
index 0000000000000000000000000000000000000000..ea1aa517a9d0cf69ad699795532feabf0fc592b0
--- /dev/null
+++ b/Source/modules/fetch/CompositeDataConsumerHandle.h
@@ -0,0 +1,59 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CompositeDataConsumerHandle_h
+#define CompositeDataConsumerHandle_h
+
+#include "public/platform/WebDataConsumerHandle.h"
+#include "wtf/OwnPtr.h"
+#include "wtf/PassOwnPtr.h"
+#include "wtf/RefPtr.h"
+
+namespace blink {
+
+// This is a utility class to construct a composite data consumer handle. It
+// owns a web data consumer handle and delegates methods. A user can update
+// the handle by using |update| method.
+class CompositeDataConsumerHandle final : public WebDataConsumerHandle {
+public:
+    // |handle| must not be null and must not be locked.
+    static PassOwnPtr<CompositeDataConsumerHandle> create(PassOwnPtr<WebDataConsumerHandle> handle)
+    {
+        ASSERT(handle);
+        return adoptPtr(new CompositeDataConsumerHandle(handle));
+    }
+    ~CompositeDataConsumerHandle() override;
+
+    // This function should be called on the thread on which this object
+    // was created. |handle| must not be null and must not be locked.
+    void update(PassOwnPtr<WebDataConsumerHandle> /* handle */);
+
+    // Utility static methods each of which provides a "basic block" of
+    // a CompositeDataConsumerHandle. For example, a user can create
+    // a CompositeDataConsumerHandle with a "waiting" handle to provide
+    // WebDataConsumerHandle APIs while waiting for another handle. When that
+    // handle arrives, the user will use |update| to replace the "waiting"
+    // handle with the arrived one.
+    //
+    // Returns a handle that returns ShouldWait for read / beginRead and
+    // UnexpectedError for endRead.
+    static PassOwnPtr<WebDataConsumerHandle> createWaitingHandle();
+
+    // Returns a handle that returns Done for read / beginRead and
+    // UnexpectedError for endRead.
+    static PassOwnPtr<WebDataConsumerHandle> createDoneHandle();
+
+private:
+    class Context;
+    class ReaderImpl;
+    Reader* obtainReaderInternal(Client*) override;
+
+    explicit CompositeDataConsumerHandle(PassOwnPtr<WebDataConsumerHandle>);
+
+    RefPtr<Context> m_context;
+};
+
+} // namespace blink
+
+#endif // CompositeDataConsumerHandle_h