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 5ca863096b36d819eac0d67cdd363465992cb0c1..b236f45330f9fc9871f3a07d0ed9a1fbb82e9a8d 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
@@ -405,10 +405,23 @@ 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) {}
+
     protected void initializeSplashScreenWidgets(int backgroundColor, Bitmap splashImage) {
         Bitmap displayIcon = splashImage == null ? mWebappInfo.icon() : splashImage;
         int minimiumSizeThreshold = getResources().getDimensionPixelSize(