Revert "CrOS: disable release notes for M78."

This reverts commit a57dd0eb.

Reason for revert: Moving release notes notification behind a feature flag instead

Original change's description:
> CrOS: disable release notes for M78.
>
> Bug: 1016946
> Change-Id: Ib9bb97271e18eb1428eded141068a4c0526a8096
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872481
> Reviewed-by: Alexander Alekseev <alemate@chromium.org>
> Commit-Queue: Yulun Wu <yulunwu@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#708776}

TBR=alemate@chromium.org,yulunwu@chromium.org

Change-Id: Ib0edac6352a69fa3a4b012055c431b37445f0f92
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1016946
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879695Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709271}

chrome/browser/chromeos/login/session/user_session_manager.cc

@@ -2453,7 +2453,16 @@ void UserSessionManager::MaybeShowU2FNotification() {
 }
 
 void UserSessionManager::MaybeShowReleaseNotesNotification(Profile* profile) {
-  return;
+  if (!base::FeatureList::IsEnabled(features::kReleaseNotes))
+    return;
+  if (!ProfileHelper::IsPrimaryProfile(profile))
+    return;
+  if (!release_notes_notification_) {
+    release_notes_notification_ =
+        std::make_unique<ReleaseNotesNotification>(profile);
+    if (chrome::GetChannel() == version_info::Channel::STABLE)
+      release_notes_notification_->MaybeShowReleaseNotes();
+  }
 }
 
 void UserSessionManager::CreateTokenUtilIfMissing() {