[ServiceWorker] Implement Headers class.

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 105 matching lines @@
       'quota/DeprecatedStorageQuota.idl',
       'quota/StorageErrorCallback.idl',
       'quota/StorageInfo.idl',
       'quota/StorageQuota.idl',
       'quota/StorageQuotaCallback.idl',
       'quota/StorageUsageCallback.idl',
       'serviceworkers/Client.idl',
       'serviceworkers/FetchEvent.idl',
       'serviceworkers/HeaderMap.idl',
       'serviceworkers/HeaderMapForEachCallback.idl',
+      'serviceworkers/Headers.idl',
+      'serviceworkers/HeadersForEachCallback.idl',
       'serviceworkers/InstallEvent.idl',
       'serviceworkers/InstallPhaseEvent.idl',
       'serviceworkers/Request.idl',
       'serviceworkers/Response.idl',
       'serviceworkers/ServiceWorker.idl',
       'serviceworkers/ServiceWorkerClients.idl',
       'serviceworkers/ServiceWorkerContainer.idl',
       'serviceworkers/ServiceWorkerGlobalScope.idl',
       'speech/SpeechGrammar.idl',
       'speech/SpeechGrammarList.idl',
@@ skipping 509 matching lines @@
       'screen_orientation/LockOrientationCallback.cpp',
       'screen_orientation/LockOrientationCallback.h',
       'screen_orientation/ScreenOrientation.cpp',
       'screen_orientation/ScreenOrientation.h',
       'screen_orientation/ScreenOrientationController.cpp',
       'screen_orientation/ScreenOrientationController.h',
       'serviceworkers/Client.cpp',
       'serviceworkers/Client.h',
       'serviceworkers/FetchEvent.cpp',
       'serviceworkers/FetchEvent.h',
+      'serviceworkers/FetchHeaderList.cpp',
+      'serviceworkers/FetchHeaderList.h',
       'serviceworkers/FetchManager.cpp',
       'serviceworkers/FetchManager.h',
       'serviceworkers/HeaderMap.cpp',
       'serviceworkers/HeaderMap.h',
       'serviceworkers/HeaderMapForEachCallback.h',
+      'serviceworkers/Headers.cpp',
+      'serviceworkers/Headers.h',
+      'serviceworkers/HeadersForEachCallback.h',
       'serviceworkers/InstallEvent.cpp',
       'serviceworkers/InstallEvent.h',
       'serviceworkers/InstallPhaseEvent.cpp',
       'serviceworkers/InstallPhaseEvent.h',
       'serviceworkers/NavigatorServiceWorker.cpp',
       'serviceworkers/NavigatorServiceWorker.h',
       'serviceworkers/RegistrationOptionList.h',
       'serviceworkers/Request.cpp',
       'serviceworkers/Request.h',
       'serviceworkers/RequestInit.h',
@@ skipping 307 matching lines @@
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketExtensionParserTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
       'websockets/WebSocketTest.cpp',
     ],
   },
 }