Do not run iOS base_unittests on macOS 10.13

Bug: 785185
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I7b432dc42efca64870effc7ff819a8036bb649da
Reviewed-on: https://chromium-review.googlesource.com/775792Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Menglu Huang <huangml@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517512}

ios/build/bots/tests/screen_size_dependent_tests.json

 {
   "tests": [
     {
-      "app": "base_unittests"
+      "app": "base_unittests",
+      "host os": "Mac-10.12"
     },
     {
       "app": "components_unittests"