Enable SwiftShader unit tests on Mac FYI try bots.

content/test/gpu/generate_buildbot_json.py

Index: content/test/gpu/generate_buildbot_json.py
diff --git a/content/test/gpu/generate_buildbot_json.py b/content/test/gpu/generate_buildbot_json.py
index c3aa4010e0197b6c5821d5239e25e96fe2249c04..800ca0accc85bdc2cb0b5595dc7d0127e3a5a0ce 100755
--- a/content/test/gpu/generate_buildbot_json.py
+++ b/content/test/gpu/generate_buildbot_json.py
@@ -1425,7 +1425,7 @@ COMMON_GTESTS = {
       {
         # Run this on the FYI waterfall and optional tryservers.
         'predicate': Predicates.FYI_AND_OPTIONAL,
-        'os_types': ['win', 'linux'],
+        'os_types': ['win', 'linux', 'mac'],
       },
     ],
     'disabled_tester_configs': [