Add OS: Mac dimension to DTK dimensions

Bug: 1138953
Change-Id: Ie8475db0b7991c867564894cccd775c0cbf19dfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476674Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Ryan Heise <heiserya@google.com>
Auto-Submit: Ryan Heise <heiserya@google.com>
Cr-Commit-Position: refs/heads/master@{#817656}

testing/buildbot/chromium.perf.json

@@ -1163,6 +1163,7 @@
           "dimension_sets": [
             {
               "cpu": "arm",
+              "os": "Mac",
               "pool": "chrome.tests.perf"
             }
           ],

tools/perf/core/perf_data_generator.py

@@ -644,6 +644,7 @@ BUILDERS = {
         'mac',
         'dimension': {
             'cpu': 'arm',
+            'os': 'Mac',
             'pool': 'chrome.tests.perf',
         },
     },