Only target 10.12.6 for webkit_layout_tests on mac_chromium_rel_ng.

We don't have enough capacity for layout tests in 10.13, and the fall
back triggering isn't working sufficiently (crbug.com/871453)

The 10.12 pool w/ 8086:0a2e GPUs has plenty of spare capacity, and
flashing them up to 10.13 isn't going to happen soon (crbug.com/873421).

Meanwhile, pending times are getting out of hand on ios-sim and
mac_chromium_rel_ng bots. So this temporarily moves layout tests
fully back to 10.12

That should free up plenty of 10.13 capacity. According to my SQL'ing,
the 10.13 pool ran ~8100 bot hours of tests yesterday; and layout tests
on mac_chromium_rel_ng accounted for ~1000 of them. (LMK if you want
my queries and I can link them.)

# No-try to help unblock mac_chromium_rel_ng
NOTRY=true

Bug: 854874
Change-Id: Ibd708f795d1df67de74cebf3f955ff175c00c0f3
Reviewed-on: https://chromium-review.googlesource.com/1175243
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583818}

testing/buildbot/chromium.mac.json

@@ -2995,12 +2995,6 @@
         }
       },
       {
-        "alternate_swarming_dimensions": [
-          {
-            "gpu": "8086:0a2e",
-            "os": "Mac-10.12.6"
-          }
-        ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [
@@ -3014,12 +3008,12 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Mac-10.13"
+              "gpu": "8086:0a2e",
+              "os": "Mac-10.12.6"
             }
           ],
           "shards": 12
-        },
-        "use_multi_dimension_trigger_script": true
+        }
       },
       {
         "isolate_name": "webkit_python_tests",

testing/buildbot/test_suite_exceptions.pyl

@@ -1622,18 +1622,13 @@
       },
       'Mac10.13 Tests': {
         # TODO(crbug.com/853356): Switch this to 10.13.
-        'use_multi_dimension_trigger_script': True,
-        'alternate_swarming_dimensions': [{
-          'gpu': '8086:0a2e',
-          'os': 'Mac-10.12.6',
-        }],
-
         # TODO(kbr): if the Swarming dimensions were explicitly specified for
         # all jobs then this wouldn't be needed.
         'swarming': {
           'dimension_sets': [
             {
-              'os': 'Mac-10.13',
+              'gpu': '8086:0a2e',
+              'os': 'Mac-10.12.6',
             },
           ],
           'shards': 12,