Add use_external_fuzzing_engine build argument

If set, the new argument allows //testing/libfuzzer/unittest_main.cc to
be used as fuzzer test driver even if use_libfuzzer and use_afl are both
false.

Change-Id: I3ce5ee97ce244ba1e09393ddf0305881688ec64d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138434Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757602}

AUTHORS

@@ -37,6 +37,7 @@ Ahmet Emir Ercin <ahmetemiremir@gmail.com>
 Ajay Berwal <a.berwal@samsung.com>
 Ajay Berwal <ajay.berwal@samsung.com>
 Ajith Kumar V <ajith.v@samsung.com>
+Akos Kiss <akiss@inf.u-szeged.hu>
 Aku Kotkavuo <a.kotkavuo@partner.samsung.com>
 Aldo Culquicondor <alculquicondor@gmail.com>
 Aleksandar Stojiljkovic <aleksandar.stojiljkovic@intel.com>

build/config/sanitizers/sanitizers.gni

@@ -79,6 +79,9 @@ declare_args() {
   # Compile for fuzzing with AFL.
   use_afl = false
 
+  # Compile for fuzzing with an external engine (e.g., Grammarinator).
+  use_external_fuzzing_engine = false
+
   # Enables core ubsan security features. Will later be removed once it matches
   # is_ubsan.
   is_ubsan_security = false
@@ -152,7 +155,7 @@ use_prebuilt_instrumented_libraries = is_msan
 # Whether we are doing a fuzzer build. Normally this should be checked instead
 # of checking "use_libfuzzer || use_afl" because often developers forget to
 # check for "use_afl".
-use_fuzzing_engine = use_libfuzzer || use_afl
+use_fuzzing_engine = use_libfuzzer || use_afl || use_external_fuzzing_engine
 
 # Args that are in turn dependent on other args must be in a separate
 # declare_args block. User overrides are only applied at the end of a