Switch debug_bots to symbol_level=1 by default.

Bug: 898161
Change-Id: I0f9f09f64318424fedad0ce3fabb116902c561aa
Reviewed-on: https://chromium-review.googlesource.com/c/1299776Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603110}

tools/mb/mb_config.pyl

@@ -1856,7 +1856,7 @@
     },
 
     'debug_bot': {
-      'mixins': ['debug', 'shared', 'goma'],
+      'mixins': ['debug', 'shared', 'goma', 'minimal_symbols'],
     },
 
     'debug_symbols_static_bot': {
@@ -1868,7 +1868,7 @@
     },
 
     'debug_trybot': {
-      'mixins': ['debug_bot', 'minimal_symbols'],
+      'mixins': ['debug_bot'],
     },
 
     'disable_nacl': {