Disable feature kSupervisedUserInitiatedExtensionInstall by default

Feature kSupervisedUserInitiatedExtensionInstall should be disabled by
default for M80. Otherwise, a child user can install extensions, but
those extensions will remain force-disabled. There will be no way to
enable those extensions, leading to a half-baked user experience.
Child users should not be able to install extensions in M80.

Bug: 1031628
Change-Id: I553d57b24fa97f3c5dba3c78a1b7acb162bcb4b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1953721Reviewed-by: Dan S <danan@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Toby Huang <tobyhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722669}

chrome/browser/supervised_user/supervised_user_features.cc

@@ -11,5 +11,5 @@ const base::Feature kSupervisedUserIframeFilter{
 
 const base::Feature kSupervisedUserInitiatedExtensionInstall{
     "SupervisedUserInitiatedExtensionInstall",
-    base::FEATURE_ENABLED_BY_DEFAULT};
+    base::FEATURE_DISABLED_BY_DEFAULT};
 }