[ios] Repurposes ios12-beta-simulator for ToT WebKit builds.

BUG=934252

Change-Id: Icf5009055afa0b4d83ece18022746c5bd373aafa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848771Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704032}

ios/build/bots/chromium.fyi/ios12-beta-simulator.json

 {
   "comments": [
-    "Run tests on iOS12beta track on 64-bit iOS 12 simulators.",
-    "Note: Xcode 11 requires OSX 10.14.3 hence 'host os'"
+    "Run tests against ToT webkit."
   ],
-  "xcode build version": "11a420a",
+  "xcode build version": "11a1027wk",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
     "is_component_build=false",