Update gpu targeted by mac-osxbeta-rel.

All of the 10.14s are on minis.

TBR=lindsayw@chromium.org

Bug: 850125
Change-Id: I50d006b529081eef229f09532d65678aabd97870
Reviewed-on: https://chromium-review.googlesource.com/c/1260308Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596475}

testing/buildbot/waterfalls.pyl

@@ -1374,8 +1374,8 @@
       },
       'mac-osxbeta-rel': {
         'swarming_mixins': [
+            'intel_iris_5100',
             'mac_10.14',
-            'no_gpu',
         ],
         'test_suites': {
           'gtest_tests': 'chromium_mac_gtests',