Update test dimensions for Mac FYI Rel GPU testers to 10.14.6

Their 3 machines were upgraded to 10.14.6 in crbug.com/1004017

TBR=ynovikov@chromium.org, kbr@chromium.org

Bug: 1003183
Change-Id: I7a50fd70bb230c146729d482586bac28965bb241
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807339
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696941}

testing/buildbot/mixins.pyl

@@ -382,10 +382,10 @@
       },
     },
   },
-  'mac_10.14.4': {
+  'mac_10.14.6': {
     'swarming': {
       'dimensions': {
-        'os': 'Mac-10.14.4',
+        'os': 'Mac-10.14.6',
       },
     },
   },

testing/buildbot/waterfalls.pyl

@@ -2804,7 +2804,7 @@
           'amd_8870m',
           'gpu_pool',
           'limited_capacity_bot',
-          'mac_10.14.4',
+          'mac_10.14.6',
         ],
         'test_suites': {
           'gtest_tests': 'gpu_fyi_mac_release_gtests',
@@ -2818,7 +2818,7 @@
           'gpu_pool',
           'intel_iris_5100',
           'limited_capacity_bot',
-          'mac_10.14.4',
+          'mac_10.14.6',
         ],
         'test_suites': {
           'gtest_tests': 'gpu_fyi_mac_release_gtests',
@@ -2830,7 +2830,7 @@
         'browser_config': 'release',
         'mixins': [
           'gpu_pool',
-          'mac_10.14.4',
+          'mac_10.14.6',
           'limited_capacity_bot',
           'nvidia_750m_mac_edition',
         ],