Shift Mac 10.12 traffic onto mac minis.

With the creation of the new 'Chrome-quarantine' pool
we accidentally left the 10.12 'Chrome' pool with no
'gpu: none' machines (i.e., no VMs) to run tasks.

This CL shifts all of the tests to physical machines
where we should have capacity.

TBR=jbudorick@chromium.org
BUG=828031
NOTRY=true

Change-Id: I1624d86887e65913330bc11b0cb1b05ddc05e1f9
Reviewed-on: https://chromium-review.googlesource.com/1103846
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567921}

testing/buildbot/waterfalls.pyl

@@ -2876,6 +2876,14 @@
         },
       },
       'Mac10.12 Tests': {
+        'swarming': {
+          'dimension_sets': [
+            {
+              'gpu': '8086:0a2e',
+              'os': 'Mac-10.12.6',
+            },
+          ],
+        },
         'test_suites': {
           'gtest_tests': 'chromium_mac_gtests',
           'isolated_scripts': 'chromium_rel_isolated_scripts',