Add Android Webview to the field trial testing config for security

intersitital dark mode

Bug: 978002
Change-Id: Ib4ca7073536c241525636decec3227817795ee64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833604Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Edward Jung (EMEA) <edwardjung@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702791}

testing/variations/fieldtrial_testing_config.json

@@ -5496,6 +5496,7 @@
         {
             "platforms": [
                 "android",
+                "android_webview",
                 "chromeos",
                 "linux",
                 "mac",