Add iOS platform to finch testing config

Bug: 978002
Change-Id: I309f3a1a4d49cbcc7369cdc86ad2c635c88a15d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919101Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Edward Jung (EMEA) <edwardjung@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715968}

testing/variations/fieldtrial_testing_config.json

@@ -5778,6 +5778,7 @@
                 "android",
                 "android_webview",
                 "chromeos",
+                "ios",
                 "linux",
                 "mac",
                 "windows"