Move WebPrerenderingSupport to platform/

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'CalculationValue.cpp',
       'CalculationValue.h',
       'Clock.cpp',
       'Clock.h',
       'ContentType.cpp',
       'ContentType.h',
       'CrossThreadCopier.cpp',
@@ skipping 186 matching lines @@
       'clipboard/ClipboardUtilitiesWin.cpp',
       'exported/Platform.cpp',
       'exported/WebAudioBus.cpp',
       'exported/WebAudioDevice.cpp',
       'exported/WebBlobData.cpp',
       'exported/WebData.cpp',
       'exported/WebActiveGestureAnimation.cpp',
       'exported/WebActiveGestureAnimation.h',
       'exported/WebHTTPBody.cpp',
       'exported/WebHTTPLoadInfo.cpp',
+      'exported/WebPrerenderingSupport.cpp',
       'exported/WebSocketStreamError.cpp',
       'exported/WebThreadSafeData.cpp',
       'exported/WebURL.cpp',
       'exported/WebURLError.cpp',
       'exported/WebURLLoadTiming.cpp',
       'exported/WebURLRequest.cpp',
       'exported/WebURLRequestPrivate.h',
       'exported/WebURLResponse.cpp',
       'exported/WebURLResponsePrivate.h',
       'fonts/FontBaseline.h',
@@ skipping 148 matching lines @@
       'geometry/RoundedRectTest.cpp',
       'graphics/ThreadSafeDataTransportTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
       'testing/TreeTestHelpers.h',
       'text/BidiResolverTest.cpp',
       'text/DateTimeFormatTest.cpp',
     ],
   },
 }