Add 10.13 as an optional dimension for layout tests

This will allow them to try 10.13 first and fall back to 10.12 if there
aren't any hosts available.

Bug: 853356
Change-Id: I0fd82376ebb2c4e501a78360bbbc675c90ee7d34
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/1330693
Commit-Queue: Brad Hall <bradhall@google.com>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609743}

testing/buildbot/chromium.mac.json

@@ -4537,6 +4537,13 @@
               "os": "Mac-10.12.6"
             }
           ],
+          "optional_dimensions": {
+            "1200": [
+              {
+                "os": "Mac-10.13"
+              }
+            ]
+          },
           "shards": 12
         }
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -1442,7 +1442,6 @@
         },
       },
       'Mac10.13 Tests': {
-        # TODO(crbug.com/853356): Switch this to 10.13.
         # TODO(kbr): if the Swarming dimensions were explicitly specified for
         # all jobs then this wouldn't be needed.
         'swarming': {
@@ -1453,6 +1452,16 @@
             },
           ],
           'shards': 12,
+          # TODO(crbug.com/853356): Switch this fully to 10.13.
+          # Starting with a 20 minute expiration for now, we will reduce this
+          # later.
+          'optional_dimensions': {
+            1200: [
+              {
+                'os': 'Mac-10.13',
+              }
+            ],
+          },
         },
       },
       'Mac10.13 Tests (dbg)': {