Move WebPrerenderingSupport to platform/

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSKeyframeRule.idl',
@@ skipping 3529 matching lines @@
             'platform/chromium/support/WebDeviceMotionData.cpp',
             'platform/chromium/support/WebDeviceOrientationData.cpp',
             'platform/chromium/support/WebFileSystemCallbacks.cpp',
             'platform/chromium/support/WebFloatQuad.cpp',
             'platform/chromium/support/WebFilterKeyframe.cpp',
             'platform/chromium/support/WebMediaConstraints.cpp',
             'platform/chromium/support/WebMediaStream.cpp',
             'platform/chromium/support/WebMediaStreamSource.cpp',
             'platform/chromium/support/WebMediaStreamTrack.cpp',
             'platform/chromium/support/WebPrerender.cpp',
-            'platform/chromium/support/WebPrerenderingSupport.cpp',
             'platform/chromium/support/WebRTCConfiguration.cpp',
             'platform/chromium/support/WebRTCICECandidate.cpp',
             'platform/chromium/support/WebRTCSessionDescription.cpp',
             'platform/chromium/support/WebRTCSessionDescriptionRequest.cpp',
             'platform/chromium/support/WebRTCStatsRequest.cpp',
             'platform/chromium/support/WebRTCStatsResponse.cpp',
             'platform/chromium/support/WebRTCVoidRequest.cpp',
             'platform/chromium/support/WebScrollbarImpl.cpp',
             'platform/chromium/support/WebScrollbarImpl.h',
             'platform/chromium/support/WebScrollbarThemeClientImpl.cpp',
@@ skipping 71 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }