gn: Tweak precompiled_header docs.

tools/gn/ninja_binary_target_writer_unittest.cc

Index: tools/gn/ninja_binary_target_writer_unittest.cc
diff --git a/tools/gn/ninja_binary_target_writer_unittest.cc b/tools/gn/ninja_binary_target_writer_unittest.cc
index 381eb39c03b9c206726a1de574ca3e796867b2a4..6ac814606f51dadc7c86dbe55b1a6dca96baae47 100644
--- a/tools/gn/ninja_binary_target_writer_unittest.cc
+++ b/tools/gn/ninja_binary_target_writer_unittest.cc
@@ -772,7 +772,6 @@ TEST(NinjaBinaryTargetWriter, GCCPrecompiledHeaders) {
   {
     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");