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

testing/libfuzzer/efficient_fuzzer.md

 # Efficient Fuzzer
 
 This document describes ways to determine your fuzzer efficiency and ways 
 to improve it.
 
 ## Overview
 
 Being a coverage-driven fuzzer, libFuzzer considers a certain input *interesting*
 if it results in new coverage. The set of all interesting inputs is called 
 *corpus*. 
@@ skipping 71 matching lines @@
 should contain hundreds (if not thousands) of items.
 
 Too small corpus size indicates some code barrier that
 libFuzzer is having problems penetrating. Common cases include: checksums,
 magic numbers etc. The easiest way to diagnose this problem is to generate a 
 [coverage report](#Coverage). To fix the issue you can:
 
 * change the code (e.g. disable crc checks while fuzzing)
 * prepare [corpus seed](#Corpus-Seed)
 * prepare [fuzzer dictionary](#Fuzzer-Dictionary)
+* specify [custom options](#Custom-Options)
 
 ## Coverage
 
 You can easily generate source-level coverage report for a given corpus:
 
 ```
 ASAN_OPTIONS=html_cov_report=1:sancov_path=./third_party/llvm-build/Release+Asserts/bin/sancov \
   ./out/libfuzzer/my_fuzzer -runs=0 ~/tmp/my_fuzzer_corpus
 ```
 
@@ skipping 58 matching lines @@
 fuzzer_test("my_protocol_fuzzer") {
   ...
   dict = "protocol.dict"
 }
 ```
 
 Make sure to submit dictionary file to git. The dictionary will be used
 automatically by ClusterFuzz once it picks up new fuzzer version (once a day).
 
 
+### Custom Options
+
+It is possible to specify [libFuzzer parameters](http://llvm.org/docs/LibFuzzer.html#usage)
+for any fuzzer being run at ClusterFuzz. Custom options will overwrite default
+values provided by ClusterFuzz.
+
+Just list all parameters in `libfuzzer_options` variable of build target:
+
+```
+fuzzer_test("my_protocol_fuzzer") {
+  ...
+  libfuzzer_options = [
+    "max_len=2048",
+    "use_traces=1",
+  ]
+}
+```
+
+Please note that `dict` parameter should be provided [separately](#Fuzzer-Dictionary).
+Other options may be passed through `libfuzzer_options` property.
+
+
 [ClusterFuzz status]: ./clusterfuzz.md#Status-Links
 [upload corpus to ClusterFuzz]: ./clusterfuzz.md#Upload-Corpus
 [AFL]: http://lcamtuf.coredump.cx/afl/