Disable RDB for telemetry_unittests on mac 10.14 tester.

Appears to be causing shard #2 to timeout consistently.

Bug: 1147992
Change-Id: I3e680dc18499c261b013f8b2cb04911e7eee5daf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533202Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826880}

testing/buildbot/chromium.mac.json

@@ -10767,7 +10767,7 @@
         },
         "name": "telemetry_unittests",
         "resultdb": {
-          "enable": true
+          "enable": false
         },
         "swarming": {
           "can_use_on_swarming_builders": true,

testing/buildbot/test_suite_exceptions.pyl

@@ -2630,6 +2630,11 @@
   },
   'telemetry_unittests': {
     'modifications': {
+      'Mac10.14 Tests': {
+        'resultdb': {
+          'enable': False,  # crbug.com/1147992
+        },
+      },
       'Win10 Tests x64 (dbg)': {
         'experiment_percentage': 100,  # crbug.com/870673
       },