[libfuzzer] store custom options in .GN build target instead of a separate file.

testing/libfuzzer/reference.md

Index: testing/libfuzzer/reference.md
diff --git a/testing/libfuzzer/reference.md b/testing/libfuzzer/reference.md
index 8f373ef79046287d4f2d899eaf829d346dec7e63..df4fd3414c6730f1ccd10a3dbe6787b23f5e27f8 100644
--- a/testing/libfuzzer/reference.md
+++ b/testing/libfuzzer/reference.md
@@ -40,15 +40,19 @@ Most common flags are:
 A fuller list of options can be found at [libFuzzer Usage] page and by running
 the binary with `-help=1`.
 
-To specify these options for ClusterFuzz, create `<my_fuzzer>.options` file:
+To specify these options for ClusterFuzz, list all parameters in
+`libfuzzer_options` target attribute:
 
 ```
-[libfuzzer]
-max_len=500
+fuzzer_test("my_fuzzer") {
+  ...
+  libfuzzer_options = [
+    "max_len=2048",
+    "use_traces=1",
+  ]
+}
 ```
 
-and specify the file in `libfuzzer_options` target attribute.
-
 [libFuzzer Usage]: http://llvm.org/docs/LibFuzzer.html#usage