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

testing/libfuzzer/reference.md

 # libFuzzer Integration Reference
 
 ## fuzzer_test GN Template
 
 Use `fuzzer_test` to define libFuzzer targets:
 
 ```
 fuzzer_test("my_fuzzer") {
   ...
 }
@@ skipping 22 matching lines @@
 Most common flags are:
 
 | Flag | Description |
 |------|-------------|
 | max_len | Maximum length of test input. |
 | timeout | Timeout of seconds. Units slower than this value will be reported as bugs. |
 
 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