Update host_os for WebRTC Chromium FYI ios-sim tests.

This should fix the current issue:
  test_runner.XcodeVersionNotFoundError: Xcode version not found: 12a8189n

Bug: None
Change-Id: I7444ed02b44225c7eb8fb83861dd082512c5f304
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2403241Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805786}

ios/build/bots/chromium.webrtc.fyi/WebRTC Chromium FYI ios-simulator.json

@@ -24,21 +24,21 @@
       "include": "webrtc_tests.json",
       "device type": "iPhone 6s Plus",
       "os": "12.4",
-      "host os": "Mac-10.14.6",
+      "host os": "Mac-10.15",
       "pool": "chromium.tests"
     },
     {
       "include": "webrtc_tests.json",
       "device type": "iPhone 6s",
       "os": "12.4",
-      "host os": "Mac-10.14.6",
+      "host os": "Mac-10.15",
       "pool": "chromium.tests"
     },
     {
       "include": "webrtc_tests.json",
       "device type": "iPad Air 2",
       "os": "12.4",
-      "host os": "Mac-10.14.6",
+      "host os": "Mac-10.15",
       "pool": "chromium.tests"
     }
   ]