Exclude CrOS from per-agent scheduling experiment.

Bug: 1090251,1113134
Change-Id: I4d2efea46316d87332e700fe4dccd2aabfa87149
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340569Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Tal Pressman <talp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795374}

testing/variations/fieldtrial_testing_config.json

@@ -1114,7 +1114,6 @@
             "platforms": [
                 "android",
                 "android_weblayer",
-                "chromeos",
                 "ios",
                 "linux",
                 "mac",