Reland "Add fieldtrial testing config for ButterForPasswords"

This reverts commit d7c5c1d5.

Reason for reland: Mac flakiness was hopefully resolved by
https://crrev.com/c/2463562

Original change's description:
> Revert "Add fieldtrial testing config for ButterForPasswords"
>
> This reverts commit 8e4ad533.
>
> Reason for revert: crbug.com/1136758 test failures on macOS
>
> Original change's description:
> > Add fieldtrial testing config for ButterForPasswords
> >
> > Bug: 991510
> > Change-Id: I447e99f11c322478f007557ab268823ed2616042
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2455446
> > Commit-Queue: Jesse Doherty <jwd@chromium.org>
> > Auto-Submit: Marc Treib <treib@chromium.org>
> > Reviewed-by: Jesse Doherty <jwd@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#815355}
>
> TBR=jwd@chromium.org,treib@chromium.org
>
> Change-Id: I41a6d02cb9dce80e5bc41e087c53f8b6b5be0335
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 991510
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462884
> Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
> Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#815507}

TBR=jwd@chromium.org
since this is unchanged from the previous version

# Not skipping CQ checks because this is a reland.

Bug: 991510
Change-Id: Id09347649546b2d7ca102ba839be17fe8cd13166
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462081Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816534}

testing/variations/fieldtrial_testing_config.json

@@ -1262,6 +1262,29 @@
             ]
         }
     ],
+    "ButterForPasswords": [
+        {
+            "platforms": [
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "ButterForPasswords",
+                    "params": {
+                        "event_1": "name:passwords_account_storage_unselected;comparator:==0;window:180;storage:180",
+                        "event_trigger": "name:passwords_account_storage_trigger;comparator:<5;window:180;storage:180",
+                        "event_used": "name:passwords_account_storage_used;comparator:==0;window:180;storage:180"
+                    },
+                    "enable_features": [
+                        "EnablePasswordsAccountStorage",
+                        "IPH_PasswordsAccountStorage"
+                    ]
+                }
+            ]
+        }
+    ],
     "CSSMatchedPropertiesCacheDependencies": [
         {
             "platforms": [