Remove WebWaitableEvent and replace it with WaitableEvent.

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'ContentDecryptionModuleResult.h',
@@ skipping 119 matching lines @@
       'Timer.cpp',
       'Timer.h',
       'TraceEvent.h',
       'TraceEventCommon.h',
       'TracedValue.cpp',
       'TracedValue.h',
       'UUID.cpp',
       'UUID.h',
       'UserGestureIndicator.cpp',
       'UserGestureIndicator.h',
+      'WaitableEvent.cpp',
+      'WaitableEvent.h',
       'WebScheduler.cpp',
       'WebTaskRunner.cpp',
       'WebThreadSupportingGC.cpp',
       'WebThreadSupportingGC.h',
       'WebThread.cpp',
       'Widget.cpp',
       'Widget.h',
       'WindowsKeyboardCodes.h',
       'animation/AnimationUtilities.h',
       'animation/CubicBezierControlPoints.cpp',
@@ skipping 998 matching lines @@
       ['OS!="mac"',
         {
           'platform_test_files': [
             'scroll/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }