MB: Enable optimize_for_fuzzing=true for libfuzzer builds.

Build with this enabled since that's what Chromium bots do:
https://cs.chromium.org/chromium/src/build/config/compiler/BUILD.gn?rcl=0&l=89

BUG=chromium:677261
R=pbos@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2604003002
Cr-Commit-Position: refs/heads/master@{#15838}
diff --git a/tools-webrtc/mb/mb_config.pyl b/tools-webrtc/mb/mb_config.pyl
index 9636300..b310d1c 100644
--- a/tools-webrtc/mb/mb_config.pyl
+++ b/tools-webrtc/mb/mb_config.pyl
@@ -239,7 +239,7 @@
       'gn', 'openh264', 'release', 'static', 'goma', 'x64'
     ],
     'libfuzzer_asan_release_bot_x64': [
-      'libfuzzer', 'asan', 'openh264_release_bot', 'x64'
+      'libfuzzer', 'asan', 'optimize_for_fuzzing', 'openh264_release_bot', 'x64'
     ],
     'memcheck_release_bot_x64': [
       'memcheck', 'gn', 'openh264_release_bot', 'x64'
@@ -425,6 +425,10 @@
       'mixins': ['openh264', 'release_bot'],
     },
 
+    'optimize_for_fuzzing': {
+      'gn_args': 'optimize_for_fuzzing=true',
+    },
+
     'release': {
       'gn_args': 'is_debug=false',
     },