[2a] Implement FetchDataLoader

Source/modules/modules.gypi

 {
   'includes': [
     '../core/core_generated.gypi',
     'modules_generated.gypi',
   ],
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
@@ skipping 730 matching lines @@
       'encryptedmedia/SimpleContentDecryptionModuleResultPromise.h',
       'fetch/CompositeDataConsumerHandle.cpp',
       'fetch/CompositeDataConsumerHandle.h',
       'fetch/DataConsumerHandleUtil.cpp',
       'fetch/DataConsumerHandleUtil.h',
       'fetch/Body.cpp',
       'fetch/Body.h',
       'fetch/BodyStreamBuffer.cpp',
       'fetch/BodyStreamBuffer.h',
       'fetch/FetchDataConsumerHandle.h',
+      'fetch/FetchDataLoader.cpp',
+      'fetch/FetchDataLoader.h',
       'fetch/FetchHeaderList.cpp',
       'fetch/FetchHeaderList.h',
       'fetch/FetchManager.cpp',
       'fetch/FetchManager.h',
       'fetch/FetchRequestData.cpp',
       'fetch/FetchRequestData.h',
       'fetch/FetchResponseData.cpp',
       'fetch/FetchResponseData.h',
       'fetch/GlobalFetch.cpp',
       'fetch/GlobalFetch.h',
@@ skipping 691 matching lines @@
       'webaudio/testing/InternalsWebAudio.cpp',
     ],
     'modules_unittest_files': [
       'accessibility/AXObjectTest.cpp',
       'cachestorage/CacheTest.cpp',
       'compositorworker/CompositorWorkerManagerTest.cpp',
       'fetch/BodyStreamBufferTest.cpp',
       'fetch/CompositeDataConsumerHandleTest.cpp',
       'fetch/DataConsumerHandleTestUtil.h',
       'fetch/DataConsumerHandleUtilTest.cpp',
+      'fetch/FetchDataLoaderTest.cpp',
       'fetch/FetchResponseDataTest.cpp',
       'fetch/RequestTest.cpp',
       'fetch/ResponseTest.cpp',
       'filesystem/DOMFileSystemBaseTest.cpp',
       'indexeddb/IDBKeyPathTest.cpp',
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'serviceworkers/ServiceWorkerContainerTest.cpp',
       'webaudio/AudioBasicProcessorHandlerTest.cpp',
       'webaudio/ConvolverNodeTest.cpp',
       'webaudio/DynamicsCompressorNodeTest.cpp',
       'webaudio/ScriptProcessorNodeTest.cpp',
       'webaudio/StereoPannerNodeTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }