Add WebLayer to ImprovedCookieControls finch experiment

This flag enables third party cookie management in the page info bubble
on WebLayer.

Bug: 1071775
Change-Id: If41e96859475bf7790ccdc6534cf0190863892de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2258859
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781608}

testing/variations/fieldtrial_testing_config.json

@@ -3299,7 +3299,8 @@
     "ImprovedCookieControls": [
         {
             "platforms": [
-                "android"
+                "android",
+                "android_weblayer"
             ],
             "experiments": [
                 {