gn: Tweak precompiled_header docs.

tools/gn/ninja_binary_target_writer_unittest.cc

 // Copyright (c) 2013 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.
 
 #include "tools/gn/ninja_binary_target_writer.h"
 
 #include <sstream>
 #include <utility>
 
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 754 matching lines @@
         "build withpch/obj/foo/no_pch_target.stamp: "
                "withpch_stamp withpch/obj/foo/no_pch_target.input1.o "
                "withpch/obj/foo/no_pch_target.input2.o\n";
     EXPECT_EQ(no_pch_expected, out.str());
   }
 
   // This target specifies PCH.
   {
     Target pch_target(&pch_settings,
                       Label(SourceDir("//foo/"), "pch_target"));
-    pch_target.config_values().set_precompiled_header("build/precompile.h");
     pch_target.config_values().set_precompiled_source(
         SourceFile("//build/precompile.h"));
     pch_target.config_values().cflags_c().push_back("-std=c99");
     pch_target.set_output_type(Target::SOURCE_SET);
     pch_target.visibility().SetPublic();
     pch_target.sources().push_back(SourceFile("//foo/input1.cc"));
     pch_target.sources().push_back(SourceFile("//foo/input2.c"));
     pch_target.SetToolchain(&pch_toolchain);
     ASSERT_TRUE(pch_target.OnResolved(&err));
 
@@ skipping 127 matching lines @@
           " | obj/foo/bar.inputs.stamp\n"
         "build obj/foo/bar.input2.o: cxx ../../foo/input2.cc"
           " | obj/foo/bar.inputs.stamp\n"
         "\n"
         "build obj/foo/bar.stamp: stamp obj/foo/bar.input1.o "
             "obj/foo/bar.input2.o\n";
 
     EXPECT_EQ(expected, out.str());
   }
 }