[libfuzzer] support multiple seed_corpus directories.

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 40 matching lines @@
 `seed_corpus` attribute to fuzzer target:
 
 ```
 fuzzer_test("my_protocol_fuzzer") {
   ...
   seed_corpus = "src/fuzz/testcases"
   ...
 }
 ```
 
+You may specify multiple seed corpus directories via `seed_corpuses` attribute:
+
+```
+fuzzer_test("my_protocol_fuzzer") {
+  ...
+  seed_corpuses = [ "src/fuzz/testcases", "src/unittest/data" ]
+  ...
+}
+```
+
+All files found in the directories and their subdirectories will be archived
+into `%YOUR_FUZZER_NAME%_seed_corpus.zip` output archive.
+
 If you don't want to store seed corpus in Chromium repository, you can upload
 corpus to Google Cloud Storage bucket used by ClusterFuzz:
 
 
 1) go to [Corpus GCS Bucket]
 
 2) open directory named `%YOUR_FUZZER_NAME%_static`
 
 3) upload corpus files into the directory
 
@@ skipping 174 matching lines @@
 ```
 
 Please note that `dict` parameter should be provided [separately](#Fuzzer-Dictionary).
 Other options may be passed through `libfuzzer_options` property.
 
 
 [AFL]: http://lcamtuf.coredump.cx/afl/
 [ClusterFuzz status]: clusterfuzz.md#Status-Links
 [Corpus GCS Bucket]: https://goto.google.com/libfuzzer-clusterfuzz-corpus
 [issue 638836]: https://bugs.chromium.org/p/chromium/issues/detail?id=638836