Move PlatformEvent.cpp/.h to Source/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 2606 matching lines @@
             'platform/LifecycleNotifier.h',
             'platform/LifecycleObserver.cpp',
             'platform/LifecycleObserver.h',
             'platform/MIMETypeFromURL.cpp',
             'platform/MIMETypeFromURL.h',
             'platform/ParsingUtilities.h',
             'platform/Partitions.cpp',
             'platform/Partitions.h',
             'platform/Pasteboard.cpp',
             'platform/Pasteboard.h',
-            'platform/PlatformEvent.cpp',
-            'platform/PlatformEvent.h',
             'platform/PlatformInstrumentation.cpp',
             'platform/PlatformInstrumentation.h',
             'platform/PlatformSpeechSynthesisUtterance.cpp',
             'platform/PlatformSpeechSynthesisUtterance.h',
             'platform/PlatformSpeechSynthesisVoice.cpp',
             'platform/PlatformSpeechSynthesisVoice.h',
             'platform/PlatformSpeechSynthesizer.cpp',
             'platform/PlatformSpeechSynthesizer.h',
             'platform/PlatformTouchEvent.h',
             'platform/PlatformTouchPoint.h',
@@ skipping 1202 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++"'
               }],
          ],
     }
 }