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 3d2b1cdfb4a0722f35a48d56d4693ca4a4035e87..9b2be15d6fd4ad94c1e59823a9535aa9870f398b 100644
--- a/Source/modules/fetch/Body.cpp
+++ b/Source/modules/fetch/Body.cpp
@@ -113,8 +113,8 @@ 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);
         *dataLost = m_queueCount != tmp_queue.size();
         while (!tmp_queue.isEmpty()) {
             std::pair<RefPtr<DOMArrayBuffer>, size_t> data = tmp_queue.takeFirst();
@@ -377,9 +377,11 @@ void Body::stop()
 
 bool Body::hasPendingActivity() const
 {
+    if (executionContext()->activeDOMObjectsAreStopped())
+        return false;
     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;
 }