Add android_webview to ServiceWorkerServicification fieldtrial testing config.

It was just not added earlier as an oversight. Shipping this feature
would affect android webview.

Bug: 881225
Change-Id: Ib101582496db28bbd23239ce71abd547faadcf22
Reviewed-on: https://chromium-review.googlesource.com/1235474Reviewed-by: Robert Kaplow (sloooow) <rkaplow@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592970}

testing/variations/fieldtrial_testing_config.json

@@ -3941,6 +3941,7 @@
         {
             "platforms": [
                 "android",
+                "android_webview",
                 "chromeos",
                 "linux",
                 "mac",