[Fetch API] Start reading blob only when the reader wants to read contents.

third_party/WebKit/Source/modules/fetch/FetchBlobDataConsumerHandle.cpp

 // 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.
 
 #include "modules/fetch/FetchBlobDataConsumerHandle.h"
 
 #include "core/dom/ExecutionContext.h"
 #include "core/fetch/FetchInitiatorTypeNames.h"
 #include "core/loader/ThreadableLoaderClient.h"
 #include "modules/fetch/CompositeDataConsumerHandle.h"
@@ skipping 173 matching lines @@
         ReaderImpl(Client* client, PassRefPtr<ReaderContext> readerContext, std::unique_ptr<WebDataConsumerHandle::Reader> reader)
             : m_readerContext(readerContext)
             , m_reader(std::move(reader))
             , m_notifier(client) { }
         ~ReaderImpl() override { }
 
         Result read(void* data, size_t size, Flags flags, size_t* readSize) override
         {
             if (m_readerContext->drained())
                 return Done;
-            m_readerContext->ensureStartLoader();
             Result r = m_reader->read(data, size, flags, readSize);
             if (!(size == 0 && (r == Ok || r == ShouldWait))) {
                 // We read non-empty data, so we cannot use the blob data
                 // handle which represents the whole data.
                 m_readerContext->clearBlobDataHandleForDrain();
+                m_readerContext->ensureStartLoader();
             }
             return r;
         }
 
         Result beginRead(const void** buffer, Flags flags, size_t* available) override
         {
             if (m_readerContext->drained())
                 return Done;
             m_readerContext->ensureStartLoader();
             m_readerContext->clearBlobDataHandleForDrain();
@@ skipping 97 matching lines @@
 
     return wrapUnique(new FetchBlobDataConsumerHandle(executionContext, blobDataHandle, new DefaultLoaderFactory));
 }
 
 std::unique_ptr<FetchDataConsumerHandle::Reader> FetchBlobDataConsumerHandle::obtainFetchDataReader(Client* client)
 {
     return m_readerContext->obtainReader(client);
 }
 
 } // namespace blink