Use minimal symbols on Cast Audio Linux.

is_chromecast=true builds default to symbol_level=2, even in release.
this seems gratuitous on the Cast Audio Linux builder.

Bug: 790121
Change-Id: Ifbce85025741ed4d85f699aef1b69c6de97e47a3
Reviewed-on: https://chromium-review.googlesource.com/1044776Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556229}

tools/mb/mb_config.pyl

@@ -1045,7 +1045,7 @@
     ],
 
     'cast_audio_release_bot': [
-      'cast', 'cast_audio', 'release_bot',
+      'cast', 'cast_audio', 'release_bot', 'minimal_symbols',
     ],
 
     'cast_audio_release_trybot': [