[libfuzzer] enable owners file generation only in use_fuzzing_engine

Owners file generation took more than 25% of full build times in CQ.
Let me enable that only in fuzzing engine build.

Bug: 995151
Change-Id: I4f490ef946ddeb729991cafbf21dd21bb5059920
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761889
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688522}

testing/libfuzzer/fuzzer_test.gni

@@ -24,7 +24,6 @@ import("//testing/test.gni")
 # - ubsan_options - UndefinedBehaviorSanitizer options.
 # - seed_corpus - a directory with seed corpus.
 # - seed_corpus_deps - dependencies for generating the seed corpus.
-# - skip_owners - if true, skips writing the owners file.
 #
 # If use_libfuzzer gn flag is defined, then proper fuzzer would be build.
 # Without use_libfuzzer or use_afl a unit-test style binary would be built on
@@ -155,7 +154,9 @@ template("fuzzer_test") {
       test_deps += [ ":" + config_file_name ]
     }
 
-    # Generate .owners file (if skip_owners is not true).
+    if (use_fuzzing_engine) {
+      # Generating owners files is slow, only enable when fuzzing engine is
+      # used.
       owners_file_name = target_name + ".owners"
       action(owners_file_name) {
         script = "//testing/libfuzzer/gen_fuzzer_owners.py"
@@ -184,6 +185,7 @@ template("fuzzer_test") {
         ]
       }
       test_deps += [ ":" + owners_file_name ]
+    }
 
     test(target_name) {
       forward_variables_from(invoker,