Enable optimize_for_fuzzing GN flag for libFuzzer ASan Debug configuration.

The flag is being used for all other configurations for a while. Looks like
we've forgotten to add it for the Debug one when it's been implemented.

Bug: 852391
Change-Id: I2f3fbd4639fdb81b304f0abdd0c8fe6f3d1dbcb3
Reviewed-on: https://chromium-review.googlesource.com/1100964Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567318}

tools/mb/mb_config.pyl

@@ -1237,7 +1237,7 @@
     ],
 
     'debug_libfuzzer_asan': [
-      'debug', 'libfuzzer', 'asan', 'chromeos_codecs', 'pdf_xfa', 'disable_nacl',
+      'debug', 'libfuzzer', 'asan', 'chromeos_codecs', 'pdf_xfa', 'disable_nacl', 'optimize_for_fuzzing',
     ],
 
     'debug_trybot': [