Roll osxbeta bot to osx 10.15

Bug: 984173
Change-Id: I34a93ba8b456cd2bf49270f55dfa00297a547168
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728185Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Lindsay Pasricha <lindsayw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682984}

testing/buildbot/mixins.pyl

@@ -368,10 +368,10 @@
       },
     },
   },
-  'mac_10.14_beta': {
+  'mac_10.15_beta': {
     'swarming': {
       'dimensions': {
-        'os': 'Mac-10.14.6',
+        'os': 'Mac-10.15',
       },
     },
   },

testing/buildbot/waterfalls.pyl

@@ -1851,7 +1851,7 @@
         'mixins': [
             'intel_iris_5100',
             'limited_capacity_bot',
-            'mac_10.14_beta',
+            'mac_10.15_beta',
         ],
         'test_suites': {
           'gtest_tests': 'chromium_mac_gtests',