Remove expiry from PageLoadDueToExternalApp.

The metric isn't used frequently, but when needed it can help see
trends in incoming traffic metrics.

BUG=975882

Change-Id: Ic3e1e2a3b3ad273a4d8d128f82068142f7315950
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713231Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717722}

tools/metrics/histograms/histograms.xml

@@ -73741,7 +73741,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="MobileIntent.PageLoadDueToExternalApp" enum="ClientAppId"
-    expires_after="2020-04-05">
+    expires_after="never">
+<!-- expires-never: Navigation (external-apps) heartbeat metric for Android. -->
+
   <owner>tedchoc@chromium.org</owner>
   <summary>
     Android: Count of page loads started by intents from external apps.