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

third_party/WebKit/Source/modules/fetch/FetchFormDataConsumerHandleTest.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/FetchFormDataConsumerHandle.h"
 
 #include "core/dom/DOMTypedArray.h"
 #include "core/html/FormData.h"
 #include "core/loader/MockThreadableLoader.h"
 #include "core/loader/ThreadableLoaderClient.h"
@@ skipping 383 matching lines @@
 TEST_F(FetchFormDataConsumerHandleTest, ZeroByteReadDoesNotAffectDrainingForComplexFormData)
 {
     std::unique_ptr<ReplayingHandle> src = ReplayingHandle::create();
     src->add(Command(Command::Data, "bar"));
     src->add(Command(Command::Done));
     std::unique_ptr<FetchDataConsumerHandle> handle = FetchFormDataConsumerHandle::createForTest(getDocument(), complexFormData(), new LoaderFactory(std::move(src)));
     std::unique_ptr<FetchDataConsumerHandle::Reader> reader = handle->obtainFetchDataReader(nullptr);
     size_t readSize;
     EXPECT_EQ(kShouldWait, reader->read(nullptr, 0, kNone, &readSize));
     testing::runPendingTasks();
-    EXPECT_EQ(kOk, reader->read(nullptr, 0, kNone, &readSize));
     RefPtr<EncodedFormData> formData = reader->drainAsFormData();
     ASSERT_TRUE(formData);
     EXPECT_TRUE(formData->isSafeToSendToAnotherThread());
     verifyComplexFormData(formData.get());
 }
 
 TEST_F(FetchFormDataConsumerHandleTest, OneByteReadAffectsDrainingForComplexFormData)
 {
     std::unique_ptr<ReplayingHandle> src = ReplayingHandle::create();
     src->add(Command(Command::Data, "bar"));
@@ skipping 22 matching lines @@
     EXPECT_EQ(kShouldWait, reader->beginRead(&buffer, kNone, &available));
     testing::runPendingTasks();
     EXPECT_EQ(kOk, reader->beginRead(&buffer, kNone, &available));
     EXPECT_FALSE(reader->drainAsFormData());
     reader->endRead(0);
     EXPECT_FALSE(reader->drainAsFormData());
 }
 
 } // namespace
 } // namespace blink