Add windows to mixed forms testing config

Change-Id: Ic473c2bdc1e06214c292afcf9c2e25c1d5d93830
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417449
Auto-Submit: Carlos IL <carlosil@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808492}

testing/variations/fieldtrial_testing_config.json

@@ -4440,6 +4440,7 @@
     "MixedFormWarnings": [
         {
             "platforms": [
+                "windows",
                 "android",
                 "chromeos",
                 "linux",