Run iOS chromium.webrtc.fyi on iOS 12.1.

This is needed after [1].

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/1706506

Bug: 976348
Change-Id: I62ff634cf34a8358a28d9085c3b6adc7f48e3599
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715290
Auto-Submit: Mirko Bonadei <mbonadei@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680405}

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

@@ -22,21 +22,21 @@
     {
       "include": "webrtc_tests.json",
       "device type": "iPhone 6s Plus",
-      "os": "11.4",
+      "os": "12.1",
       "host os": "Mac-10.13.6",
       "pool": "Chrome"
     },
     {
       "include": "webrtc_tests.json",
       "device type": "iPhone 6s",
-      "os": "11.4",
+      "os": "12.1",
       "host os": "Mac-10.13.6",
       "pool": "Chrome"
     },
     {
       "include": "webrtc_tests.json",
       "device type": "iPad Air 2",
-      "os": "11.4",
+      "os": "12.1",
       "host os": "Mac-10.13.6",
       "pool": "Chrome"
     }