Make mac-cocoa-rel explicitly target 10.13.

was implicit previously, leading to the weird "... on Linux" step
text on browser_tests.

TBR=martiniss@chromium.org

Bug: 855301
Change-Id: I06e927a6c60f0d8d296370a756496a73e0aa03b1
Reviewed-on: https://chromium-review.googlesource.com/1121831Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572006}

testing/buildbot/chromium.mac.json

@@ -3294,6 +3294,7 @@
           "dimension_sets": [
             {
               "gpu": "none",
+              "os": "Mac-10.13",
               "pool": "Chrome-quarantine"
             }
           ],
@@ -3306,7 +3307,12 @@
           "--disable-features=ViewsBrowserWindows"
         ],
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Mac-10.13"
+            }
+          ]
         },
         "test": "components_browsertests"
       },
@@ -3315,7 +3321,12 @@
           "--disable-features=ViewsBrowserWindows"
         ],
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Mac-10.13"
+            }
+          ]
         },
         "test": "content_browsertests"
       },
@@ -3325,6 +3336,11 @@
         ],
         "swarming": {
           "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Mac-10.13"
+            }
+          ],
           "shards": 3
         },
         "test": "interactive_ui_tests"

testing/buildbot/waterfalls.pyl

@@ -2881,6 +2881,13 @@
         ],
       },
       'mac-cocoa-rel': {
+        'swarming': {
+          'dimension_sets': [
+            {
+              'os': 'Mac-10.13',
+            },
+          ],
+        },
         'test_suites': {
           'gtest_tests': 'chromium_browser_tests',
         },