Remove PlatformEvent it is no longer used.

third_party/WebKit/Source/platform/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//third_party/WebKit/public/public_features.gni")
 import("//third_party/WebKit/Source/build/scripts/scripts.gni")
@@ skipping 273 matching lines @@
     "MemoryCoordinator.cpp",
     "MemoryCoordinator.h",
     "PODArena.h",
     "PODFreeListArena.h",
     "PODInterval.h",
     "PODIntervalTree.h",
     "PODRedBlackTree.h",
     "PartitionAllocMemoryDumpProvider.cpp",
     "PartitionAllocMemoryDumpProvider.h",
     "PasteMode.h",
-    "PlatformEvent.h",
     "PlatformInstrumentation.cpp",
     "PlatformInstrumentation.h",
     "PlatformResourceLoader.cpp",
     "PlatformResourceLoader.h",
     "PluginScriptForbiddenScope.cpp",
     "PluginScriptForbiddenScope.h",
     "PopupMenu.h",
     "Prerender.cpp",
     "Prerender.h",
     "PrerenderClient.h",
@@ skipping 1809 matching lines @@
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:inside_blink",
   ]
 
   deps = [
     ":test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }