Move mac-chrome's sizes test from 10.14 to 10.15.

The latest build is expiring its single 'sizes' test:
https://ci.chromium.org/p/chrome/builders/ci/mac-chrome/3489

Likely cause most of the bots in the test pool on chrome-swarming are
moving to 10.15. This updates it to target 10.15.

Bug: None
Change-Id: I68435fc7d4c0a6f152af37edb1663802a34fbabf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163730Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762144}

testing/buildbot/chrome.ci.json

@@ -3880,7 +3880,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Mac-10.14.6",
+              "os": "Mac-10.15",
               "pool": "chrome.tests.template"
             }
           ],

testing/buildbot/chrome.json

@@ -2851,7 +2851,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Mac-10.14.6",
+              "os": "Mac-10.15",
               "pool": "chrome.tests.template"
             }
           ],

testing/buildbot/waterfalls.pyl

@@ -155,7 +155,7 @@
         ],
         'mixins': [
           'chrome-swarming-pool',
-          'mac_10.14',
+          'mac_10.15',
         ],
         'test_suites': {
           'isolated_scripts': 'chrome_sizes',