Add "ios" to fieldtrial testing config for SyncInstanceIDTokenTTL

Bug: 1034915
Change-Id: I1216bf20521a43d870dd40a30149d490fb41fc90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151508Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759659}

testing/variations/fieldtrial_testing_config.json

@@ -5502,6 +5502,7 @@
         {
             "platforms": [
                 "chromeos",
+                "ios",
                 "linux",
                 "mac",
                 "windows"