Move PlatformEvent.cpp/.h to Source/platform/

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'Clock.cpp',
       'Clock.h',
       'ContentType.cpp',
       'ContentType.h',
       'DateComponents.cpp',
       'DateComponents.h',
       'Decimal.cpp',
@@ skipping 13 matching lines @@
       'LinkHash.cpp',
       'LinkHash.h',
       'LocalizedStrings.cpp',
       'LocalizedStrings.h',
       'Logging.cpp',
       'Logging.h',
       'NotImplemented.cpp',
       'NotImplemented.h',
       'Partitions.cpp',
       'Partitions.h',
+      'PlatformEvent.h',
       'PlatformThreadData.cpp',
       'PlatformThreadData.h',
       'PurgeableBuffer.cpp',
       'PurgeableBuffer.h',
       'SharedBuffer.cpp',
       'SharedBuffer.h',
       'SharedBufferChunkReader.cpp',
       'SharedBufferChunkReader.h',
       'SharedTimer.cpp',
       'SharedTimer.h',
@@ skipping 77 matching lines @@
     'platform_test_files': [
       'clipboard/ClipboardUtilitiesTest.cpp',
       'text/DateTimeFormatTest.cpp',
       'ClockTest.cpp',
       'DecimalTest.cpp',
       'LayoutUnitTest.cpp',
       'SharedBufferTest.cpp',
     ],
   },
 }