[finch] Fixing platform coverage for test variations.

Added everything but webview in
https://chromium-review.googlesource.com/c/chromium/src/+/1800750,
correcting it here.

Bug: 968090

Change-Id: I0647910935eca4e53396940b0bb938045c6935d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814936Reviewed-by: Nik Bhagat <nikunjb@chromium.org>
Commit-Queue: Arthur Hemery <ahemery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700185}

testing/variations/fieldtrial_testing_config.json

@@ -4394,7 +4394,8 @@
                 "linux",
                 "mac",
                 "windows",
-                "android"
+                "android",
+                "android_webview"
             ],
             "experiments": [
                 {