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

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

Index: third_party/WebKit/Source/modules/fetch/FetchBlobDataConsumerHandle.cpp
diff --git a/third_party/WebKit/Source/modules/fetch/FetchBlobDataConsumerHandle.cpp b/third_party/WebKit/Source/modules/fetch/FetchBlobDataConsumerHandle.cpp
index c2283f909dde6225d2e574682dad4dcf708ee49c..59390959a335a000cb8765aefb19e2c83966526c 100644
--- a/third_party/WebKit/Source/modules/fetch/FetchBlobDataConsumerHandle.cpp
+++ b/third_party/WebKit/Source/modules/fetch/FetchBlobDataConsumerHandle.cpp
@@ -194,12 +194,12 @@ public:
         {
             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;
         }