Extend field trial to android_webview

This should have been included in https://crrev.com/c/2030951 already.
I missed that both "android_webview" and "ios" were missing.
The set of "android", "android_webview" and "ios" hopefully covers all
mobile platforms.

R=bcwhite@chromium.org

Bug: 1040061, 1040030
Change-Id: I8aa87b15c849be9d62c88df0a9f7eb7615eb6ec4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036056Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738483}

testing/variations/fieldtrial_testing_config.json

@@ -7177,6 +7177,7 @@
         {
             "platforms": [
                 "android",
+                "android_webview",
                 "ios"
             ],
             "experiments": [