cast: Enable compiling of EXO on cast linux bots

This is disabled by default on regular builds to conserve space. Enable
it in tests to gain coverage.

Bug: None
Test: None
Change-Id: I215b80df0c2f7594310e94acb0e6ff313dd659ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678817Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676535}

tools/mb/mb_config.pyl

@@ -1181,11 +1181,11 @@
     # Cast Linux takes very long in linking, possibly due to being on GCE
     # (crbug/794423).
     'cast_release_bot': [
-      'cast', 'release_bot', 'minimal_symbols',
+      'cast', 'cast_exo', 'release_bot', 'minimal_symbols',
     ],
 
     'cast_release_trybot': [
-      'cast', 'release_trybot',
+      'cast', 'cast_exo', 'release_trybot',
     ],
 
     'cast_audio_release_bot': [
@@ -1955,6 +1955,10 @@
       'gn_args': 'is_cast_audio_only=true'
     },
 
+    'cast_exo': {
+      'gn_args': 'enable_cast_wayland_server=true',
+    },
+
     'cfi': {
       'gn_args': 'is_cfi=true',
     },