[libFuzzer] Suppress -Wunused-result error caused by _FORTIFY_SOURCE config.

Bug: 932188
Change-Id: Ifa2342a2620a6a63beb00dda306784062271d4e6
Reviewed-on: https://chromium-review.googlesource.com/c/1474992Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632745}

third_party/libFuzzer/BUILD.gn

@@ -4,23 +4,35 @@
 
 import("///build/config/sanitizers/sanitizers.gni")
 
+config("ignore_unused_result_warning") {
+  if (is_clang) {
+    cflags = [
+      # See crbug.com/932188, libFuzzer does not check the result of write()
+      # when it does raw printing.
+      "-Wno-unused-result",
+    ]
+  }
+}
+
 # Engine should be compiled without coverage (infinite loop in trace_cmp).
 fuzzing_engine_remove_configs = [
   "//build/config/coverage:default_coverage",
   "//build/config/sanitizers:default_sanitizer_flags",
 ]
 
-# Add any sanitizer flags back.  In MSAN builds, instrumenting libfuzzer with
+# Add any sanitizer flags back. In MSAN builds, instrumenting libfuzzer with
 # MSAN is necessary since all parts of the binary need to be instrumented for it
-# to work.  ASAN builds are more subtle: libfuzzer depends on features from the
-# C++ STL.  If it were not instrumented, templates would be insantiated without
-# ASAN from libfuzzer and with ASAN in other TUs.  The linker might merge
+# to work. ASAN builds are more subtle: libfuzzer depends on features from the
+# C++ STL. If it were not instrumented, templates would be insantiated without
+# ASAN from libfuzzer and with ASAN in other TUs. The linker might merge
 # instrumented template instantiations with non-instrumented ones (which could
 # have a different ABI) in the final binary, which is problematic for TUs
-# expecting one particular ABI (https://crbug.com/915422).  The other sanitizers
+# expecting one particular ABI (https://crbug.com/915422). The other sanitizers
 # are added back for the same reason.
-fuzzing_engine_add_configs =
-    [ "//build/config/sanitizers:default_sanitizer_flags_but_coverage" ]
+fuzzing_engine_add_configs = [
+  "//build/config/sanitizers:default_sanitizer_flags_but_coverage",
+  ":ignore_unused_result_warning",
+]
 
 source_set("libfuzzer") {
   sources = [