[iOS][infra] Output iOS disabled tests from coverage builder.

Add the test arg to iOS code coverage runner so that iOS disabled tests
will be written to a file, parsed in test runner and reported to result
json file. Use coverage builder as this is relative static information
that's suitable to run on coverage CI builder.
Note: This arg currently only works for outputting disabled GTests.

Bug: 1065022
Change-Id: I6481e271b970397829f0f8301bd1089c7a0a6076
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364029Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800359}

testing/buildbot/mixins.pyl

@@ -308,6 +308,14 @@
       ],
     },
   },
+  'ios_output_disabled_tests': {
+    '$mixin_append': {
+      'args': [
+        '--args-json',
+        '{"test_args": ["--write-compiled-tests-json-to-writable-path"]}'
+      ],
+    },
+  },
   'isolate_profile_data': {
     'isolate_profile_data': True,
   },

testing/buildbot/waterfalls.pyl

@@ -2059,6 +2059,7 @@
       },
       'ios-simulator-code-coverage': {
         'mixins': [
+          'ios_output_disabled_tests',
           'isolate_profile_data',
           'mac_10.15',
           'mac_toolchain',