Change the target type of platform_wtf to source_set.

third_party/WebKit/Source/platform/wtf/BUILD.gn

 # Copyright 2017 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.
 
 # The below is a temporary setup during the WTF migration project:
 # https://groups.google.com/a/chromium.org/d/msg/blink-dev/tLdAZCTlcAA/bYXVT8gYCAAJ
 #
 # We are moving wtf/ files to platform/wtf/ incrementally, thus, conceptually,
 # the "wtf" target in wtf/BUILD.gn is being split into two, in a way that
 # only wtf/ can refer the contents in platform/wtf/.
@@ skipping 39 matching lines @@
     if (is_component_build) {
       # Chromium windows multi-dll build enables C++ exceptions and this causes
       # wtf to generate 4291 warning due to operator new/delete
       # implementations. Disable the warning for chromium windows multi-dll
       # build.
       cflags += [ "/wd4291" ]
     }
   }
 }
 
-component("platform_wtf") {
+source_set("platform_wtf") {
   sources = [
     "AddressSanitizer.h",
     "Alignment.h",
     "AutoReset.h",
     "BitwiseOperations.h",
     "ByteSwap.h",
     "CPU.h",
     "CheckedNumeric.h",
     "Compiler.h",
     "ConditionalDestructor.h",
@@ skipping 73 matching lines @@
     #   "text/AtomicStringCF.cpp",
     #   "text/StringImplCF.cpp",
     # ]
   }
 
   if (remove_webcore_debug_symbols) {
     configs -= [ "//build/config/compiler:default_symbols" ]
     configs += [ "//build/config/compiler:no_symbols" ]
   }
 }