Add a libfuzzer config to the build system. (#11921)

build/config/compiler/BUILD.gn

@@ -304,6 +304,9 @@ declare_args() {
   # enable undefined behavior sanitizer
   is_ubsan = false
 
+  # enable libfuzzer
+  is_libfuzzer = false
+
   # Exit on sanitize error. Generally standard libraries may get errors
   # so not stopping on the first error is often useful
   is_sanitize_fatal = true
@@ -374,7 +377,16 @@ config("sanitize_default") {
   }
 }
 
+config("libfuzzer_fuzzing") {
+  cflags = [ "-fsanitize=fuzzer" ]
+  ldflags = cflags
+}
+
 config("fuzzing_default") {
+  configs = []
+  if (is_libfuzzer) {
+    configs += [ ":libfuzzer_fuzzing" ]
+  }
 }
 
 declare_args() {