Add Physical Web UMA for ListUrlsActivity referers

chrome/android/java/src/org/chromium/chrome/browser/physicalweb/UrlManager.java

Index: chrome/android/java/src/org/chromium/chrome/browser/physicalweb/UrlManager.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/physicalweb/UrlManager.java b/chrome/android/java/src/org/chromium/chrome/browser/physicalweb/UrlManager.java
index 231c090c0ca647be7e317c23144a5b6c0e4eb80c..9a63059b37389d292f33cf2de49bae28257ea3eb 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/physicalweb/UrlManager.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/physicalweb/UrlManager.java
@@ -55,6 +55,8 @@ class UrlManager {
     private static final String PREFS_VERSION_KEY = "physicalweb_version";
     private static final String PREFS_NEARBY_URLS_KEY = "physicalweb_nearby_urls";
     private static final String PREFS_RESOLVED_URLS_KEY = "physicalweb_resolved_urls";
+    private static final String PREFS_NOTIFICATION_UPDATE_TIMESTAMP =
+            "physicalweb_notification_update_timestamp";
     private static final int PREFS_VERSION = 3;
     private static final long STALE_NOTIFICATION_TIMEOUT_MILLIS = 30 * 60 * 1000;
     private static UrlManager sInstance = null;
@@ -342,7 +344,25 @@ class UrlManager {
         });
     }
 
+    /**
+     * Gets the time since the last notification update.
+     * @return the elapsed realtime since the most recent notification update.
+     */
+    public long getTimeSinceNotificationUpdate() {
+        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(mContext);
+        long timestamp = prefs.getLong(PREFS_NOTIFICATION_UPDATE_TIMESTAMP, 0);
+        return SystemClock.elapsedRealtime() - timestamp;
+    }
+
     private void updateNotification(boolean isUrlListEmptyBefore, boolean isUrlListEmptyAfter) {
+        // Record a timestamp.
+        // This is useful for tracking whether a notification is pressed soon after an update or
+        // much later.
+        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(mContext);
+        SharedPreferences.Editor editor = prefs.edit();
+        editor.putLong(PREFS_NOTIFICATION_UPDATE_TIMESTAMP, SystemClock.elapsedRealtime());
+        editor.apply();
+
         if (isUrlListEmptyAfter) {
             clearNotification();
             cancelClearNotificationAlarm();