Move CrossThreadHolder to CrossThreadHolder.h

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 867 matching lines @@
       'encryptedmedia/SimpleContentDecryptionModuleResultPromise.cpp',
       '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/CrossThreadHolder.h',
       'fetch/DataConsumerTee.cpp',
       'fetch/DataConsumerTee.h',
       'fetch/FetchBlobDataConsumerHandle.cpp',
       'fetch/FetchBlobDataConsumerHandle.h',
       'fetch/FetchDataConsumerHandle.h',
       'fetch/FetchDataLoader.cpp',
       'fetch/FetchDataLoader.h',
       'fetch/FetchHeaderList.cpp',
       'fetch/FetchHeaderList.h',
       'fetch/FetchManager.cpp',
@@ skipping 869 matching lines @@
       'webaudio/AudioBasicProcessorHandlerTest.cpp',
       'webaudio/ConvolverNodeTest.cpp',
       'webaudio/DynamicsCompressorNodeTest.cpp',
       'webaudio/ScriptProcessorNodeTest.cpp',
       'webaudio/StereoPannerNodeTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }