[libfuzzer] support multiple seed_corpus directories.

testing/libfuzzer/fuzzer_test.gni

Index: testing/libfuzzer/fuzzer_test.gni
diff --git a/testing/libfuzzer/fuzzer_test.gni b/testing/libfuzzer/fuzzer_test.gni
index bfdee49c69a741e7284b5b280d36c0f2f8e45ebb..8f4ef6e296dc72239f421816ac5c54783562a583 100644
--- a/testing/libfuzzer/fuzzer_test.gni
+++ b/testing/libfuzzer/fuzzer_test.gni
@@ -37,21 +37,34 @@ template("fuzzer_test") {
       test_deps += invoker.deps
     }
 
-    if (defined(invoker.seed_corpus)) {
+    if (defined(invoker.seed_corpus) || defined(invoker.seed_corpuses)) {
+      assert(!(defined(invoker.seed_corpus) && defined(invoker.seed_corpuses)),
+             "Do not use both seed_corpus and seed_corpuses for $target_name.")
+
       out = "$root_build_dir/$target_name" + "_seed_corpus.zip"
 
       action(target_name + "_seed_corpus") {
         script = "//testing/libfuzzer/archive_corpus.py"
+
         args = [
-          "--corpus",
-          rebase_path(invoker.seed_corpus),
           "--output",
           rebase_path(out),
         ]
 
+        if (defined(invoker.seed_corpus)) {
+          args += [ rebase_path(invoker.seed_corpus) ]
+        }
+
+        if (defined(invoker.seed_corpuses)) {
+          foreach(seed_corpus_path, invoker.seed_corpuses) {
+            args += [ rebase_path(seed_corpus_path) ]
+          }
+        }
+
         outputs = [
           out,
         ]
+
         deps = [
           "//testing/libfuzzer:seed_corpus",
         ]