Disable browser_tests on Mac10.13 Tests and mac-rel

This is an emergency measure. On Jan 8th, many browser_tests were
re-enabled on Mac10.13 Tests, and were thereby also re-enabled on
the mac-rel CQ bot. This resulted in a 50% increase in browser_tests
runtime at both the median and atthe 90th percentile (approximately
2hrs to 3hrs). This has led to capacity shortages and significant
violations of the Chromium CQ cycle time SLO.

We are disabling this test suite as a simple emergency measure until
better long-term solutions (disabling just a subset of the tests,
having mac-rel mirror Mac10.14 Tests instead, or other ideas) are
in place.

Bug: 1042757
Change-Id: Iad8e9f852dccf8c630b4fe165103d25b0bc3bbd7
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015786Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
Auto-Submit: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734166}

testing/buildbot/chromium.mac.json

@@ -5029,24 +5029,6 @@
         "test": "boringssl_ssl_tests",
         "test_target": "//third_party/boringssl:boringssl_ssl_tests"
       },
-      {
-        "merge": {
-          "args": [],
-          "script": "//testing/merge_scripts/standard_gtest_merge.py"
-        },
-        "swarming": {
-          "can_use_on_swarming_builders": true,
-          "dimension_sets": [
-            {
-              "gpu": "none",
-              "os": "Mac-10.13.6"
-            }
-          ],
-          "shards": 10
-        },
-        "test": "browser_tests",
-        "test_target": "//chrome/test:browser_tests"
-      },
       {
         "merge": {
           "args": [],

testing/buildbot/test_suite_exceptions.pyl

@@ -93,6 +93,7 @@
       'linux-win_cross-rel',
       'ToTLinuxTSan',  # https://crbug.com/368525
       'Mac10.10 Tests',  # https://crbug.com/828031
+      'Mac10.13 Tests',  # https://crbug.com/1042757
       'Linux TSan Tests',  # https://crbug.com/368525
       'Win10 Tests x64 (dbg)',
     ],