Revert "Disable iframes filtering feature for supervised users"

This reverts commit 9e410191.

Reason for revert: Needed to merge this change into M85 and M86. It is not needed on Tot.

Original change's description:
> Disable iframes filtering feature for supervised users
>
> The feature has been disabled in Chrome OS < M87 with
> Finch kill-switch and in this cl we apply the same change
> in Chromium repo.
>
> Bug: 1132862
> Change-Id: I7c3446b6a6d7e4d06cdf2df5f12d6cf9eec4dc24
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435721
> Reviewed-by: Aga Wronska <agawronska@chromium.org>
> Reviewed-by: Yilkal Abe <yilkal@chromium.org>
> Auto-Submit: Aga Wronska <agawronska@chromium.org>
> Commit-Queue: Yilkal Abe <yilkal@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#811255}

TBR=agawronska@chromium.org,yilkal@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1132862
Change-Id: Ia74140aaf0510f6b6d9e3b272c02e4cfd83d74f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437401Reviewed-by: Aga Wronska <agawronska@chromium.org>
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812025}

chrome/browser/supervised_user/supervised_user_features.cc

@@ -7,7 +7,7 @@
 namespace supervised_users {
 
 const base::Feature kSupervisedUserIframeFilter{
-    "SupervisedUserIframeFilter", base::FEATURE_DISABLED_BY_DEFAULT};
+    "SupervisedUserIframeFilter", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kSupervisedUserInitiatedExtensionInstall{
     "SupervisedUserInitiatedExtensionInstall",