[libfuzzer][v8][inspector] Restrict alphabet to ascii

This is an optimization to increase efficiency of libfuzzer (by avoiding
inputs which are unlikely to be valid JS).
It's OK if other fuzzing engines still pass non-ascii input.

R=mmoroz@chromium.org
CC=ahaas@chromium.org

Bug: chromium:1142437
Change-Id: If4560b7a8c04a044835464462893caf8549f0615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518857Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824350}

testing/libfuzzer/fuzzers/BUILD.gn

@@ -151,6 +151,10 @@ fuzzer_test("v8_inspector_fuzzer") {
   ]
   dict = "dicts/generated/javascript.dict"
   seed_corpus = "//v8/test/inspector/"
+
+  # The fuzzer is able to handle any input, but since the input is interpreted
+  # as JS code, restricting to ascii only will increase fuzzing efficiency.
+  libfuzzer_options = [ "only_ascii=1" ]
   environment_variables = [ "AFL_DRIVER_DONT_DEFER=1" ]
 }