Introduce ExclusiveStreamReader.

Source/modules/fetch/Body.cpp

Index: Source/modules/fetch/Body.cpp
diff --git a/Source/modules/fetch/Body.cpp b/Source/modules/fetch/Body.cpp
index 5583293f2544c36209877a0c9cac37eab8fe1bdb..f0e675cc2244d76f4bdc371828857b4aff2af03f 100644
--- a/Source/modules/fetch/Body.cpp
+++ b/Source/modules/fetch/Body.cpp
@@ -112,8 +112,8 @@ public:
         }
         // Take back the data in |m_stream|.
         Deque<std::pair<RefPtr<DOMArrayBuffer>, size_t>> tmp_queue;
-        if (m_stream->state() == ReadableStream::Readable)
-            m_stream->read(tmp_queue);
+        if (m_stream->stateInternal() == ReadableStream::Readable)
+            m_stream->readInternal(tmp_queue);
         while (!tmp_queue.isEmpty()) {
             std::pair<RefPtr<DOMArrayBuffer>, size_t> data = tmp_queue.takeFirst();
             m_drainingStreamBuffer->write(data.first);
@@ -363,7 +363,7 @@ bool Body::hasPendingActivity() const
 {
     if (m_resolver)
         return true;
-    if (m_stream && (m_stream->state() == ReadableStream::Readable || m_stream->state() == ReadableStream::Waiting))
+    if (m_stream && m_stream->hasPendingActivity())
         return true;
     return false;
 }