Fix VerifyHTMLFetchedFromAppCacheBeforeDelay field trial config

This CL adds the android_webview platform to the feature's config.

Change-Id: I799937263359bd7a7952632c56a61514bf4d2cda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616558Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841837}

testing/variations/fieldtrial_testing_config.json

@@ -7950,7 +7950,8 @@
                 "chromeos",
                 "linux",
                 "ios",
-                "android"
+                "android",
+                "android_webview"
             ],
             "experiments": [
                 {