Add windows to platforms for ClientSideDetectionModel

The experiment is running on Windows, but the field trial testing config
was landed when there were too many configs on Windows. The temporary
workaround was to leave Windows out of the platforms. We can now undo
that temporary workaround.

Change-Id: Ifee05f28112cc8af9c87cc18816b10450f9fae44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512519Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823283}

testing/variations/fieldtrial_testing_config.json

@@ -1650,7 +1650,8 @@
             "platforms": [
                 "chromeos",
                 "linux",
-                "mac"
+                "mac",
+                "windows"
             ],
             "experiments": [
                 {