[iOS] Enable ResultDB on an iOS FYI builder.

A potential fix landed for the flaky "Connection aborted" issue in iOS
(See bug). Enable ResultDB on some fyi builders to verify the fix:
  * ios-simulator-multi-window
  * ios14-beta-simulator
  * ios14-sdk-simulator

These builders are not monitored or already have failing suites.

Bug: 1136998
Change-Id: I30577514f365c676c9e5d22143bc08b104499ca4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536932Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827459}

testing/buildbot/waterfalls.pyl

@@ -2318,6 +2318,7 @@
       },
       'ios-simulator-multi-window': {
         'mixins': [
+          'enable_resultdb',
           'mac_10.15',
           'mac_toolchain',
           'out_dir_arg',
@@ -2363,6 +2364,7 @@
           'all',
         ],
         'mixins': [
+          'enable_resultdb',
           'mac_10.15',
           'mac_toolchain',
           'out_dir_arg',
@@ -2380,6 +2382,7 @@
           'all',
         ],
         'mixins': [
+          'enable_resultdb',
           'mac_10.15',
           'mac_toolchain',
           'out_dir_arg',