ServiceWorker: register() should return the existing registration object if available

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 299 matching lines @@
       'exported/WebRTCVoidRequest.cpp',
       'exported/WebSchedulerProxy.cpp',
       'exported/WebScrollbarImpl.cpp',
       'exported/WebScrollbarImpl.h',
       'exported/WebScrollbarThemeClientImpl.cpp',
       'exported/WebScrollbarThemeClientImpl.h',
       'exported/WebScrollbarThemeGeometryNative.cpp',
       'exported/WebScrollbarThemeGeometryNative.h',
       'exported/WebSerializedOrigin.cpp',
       'exported/WebServiceWorkerProxy.cpp',
+      'exported/WebServiceWorkerRegistrationProxy.cpp',
       'exported/WebServiceWorkerRequest.cpp',
       'exported/WebServiceWorkerResponse.cpp',
       'exported/WebSocketHandshakeRequestInfo.cpp',
       'exported/WebSocketHandshakeResponseInfo.cpp',
       'exported/WebSocketStreamError.cpp',
       'exported/WebSourceInfo.cpp',
       'exported/WebSpeechSynthesisUtterance.cpp',
       'exported/WebSpeechSynthesisVoice.cpp',
       'exported/WebSpeechSynthesizerClientImpl.cpp',
       'exported/WebSpeechSynthesizerClientImpl.h',
@@ skipping 638 matching lines @@
       'graphics/ImageLayerChromiumTest.cpp',
       'graphics/OpaqueRectTrackingContentLayerDelegateTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
   },
 }