Add UMA metric to track the time elapsed since a WebAPK was last launched

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java

Index: chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java
index 6acfaacd43f8457c0f941ab1efe3e5c6332562b5..78264c4311d77f26c6a84a5ae71c29588f82187c 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java
@@ -362,10 +362,23 @@ public class WebappActivity extends FullScreenActivity {
         // app is not directly launched from the home screen, as this interferes
         // with the heuristic.
         if (mWebappInfo.isLaunchedFromHomescreen()) {
+            boolean previouslyLaunched = storage.hasBeenLaunched();
+            long previousUsageTimestamp = storage.getLastUsedTime();
+            storage.setHasBeenLaunched();
             storage.updateLastUsedTime();
+            onUpdatedLastUsedTime(storage, previouslyLaunched, previousUsageTimestamp);
         }
     }
 
+    /**
+     * Called after updating the last used time in {@link WebappDataStorage}.
+     * @param previouslyLaunched Whether the webapp has been previously launched from the home
+     *     screen.
+     * @param previousUsageTimestamp The previous time that the webapp was used.
+     */
+    protected void onUpdatedLastUsedTime(
+            WebappDataStorage storage, boolean previouslyLaunched, long previousUsageTimestamp) {}
+
     private void updateUrlBar() {
         Tab tab = getActivityTab();
         if (tab == null || mUrlBar == null) return;