Disable AFDO profile for cronet bots.

Downstream bots are not compiling due to the profile not being there.
Rather than add the profile, I think it's better to disable AFDO for
cronet for now, since there's no reason to believe that the profile
would be accurate for cronet.

Bug: 796921
Change-Id: Id8337e695f06095e037c1611fa6e9a9ba038029c
Reviewed-on: https://chromium-review.googlesource.com/842944Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526011}

tools/mb/mb_config.pyl

@@ -1841,7 +1841,8 @@
       'gn_args': ('disable_file_support=true disable_ftp_support=true '
                   'enable_websockets=false use_platform_icu_alternatives=true '
                   'use_partition_alloc=false enable_reporting=false '
-                  'include_transport_security_state_preload_list=false'),
+                  'include_transport_security_state_preload_list=false '
+                  'clang_use_default_sample_profile=false'),
     },
 
     'cros_chrome_sdk': {