Bump Chrome-GPU experimental bots to 10.13.4

R=kbr@chromium.org

Bug: 809569
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I24a5f0de433d52256d1c685824c0eebf72b104a8
Reviewed-on: https://chromium-review.googlesource.com/905268Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Bryce Albritton <dba@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535119}

content/test/gpu/generate_buildbot_json.py

@@ -610,7 +610,7 @@ FYI_WATERFALL = {
       'swarming_dimensions': [
         {
           'gpu': '8086:0a2e',
-          'os': 'Mac-10.13.2',
+          'os': 'Mac-10.13.4',
           'pool': 'Chrome-GPU',
         },
       ],
@@ -623,7 +623,7 @@ FYI_WATERFALL = {
         {
           'gpu': '1002:6821',
           'hidpi': '1',
-          'os': 'Mac-10.13.2',
+          'os': 'Mac-10.13.4',
           'pool': 'Chrome-GPU',
         },
       ],
@@ -636,7 +636,7 @@ FYI_WATERFALL = {
         {
           'gpu': '10de:0fe9',
           'hidpi': '1',
-          'os': 'Mac-10.13.2',
+          'os': 'Mac-10.13.4',
           'pool': 'Chrome-GPU',
         },
       ],