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

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

Index: chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkActivity.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkActivity.java b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkActivity.java
index c4d55b511995c8125fbede9e250d66319866d5ca..2102df9d6fb1668063677ea5c92275187252afef 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkActivity.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkActivity.java
@@ -182,6 +182,16 @@ public class WebApkActivity extends WebappActivity {
         maybeShowDisclosure(storage);
     }
 
+    @Override
+    protected void onWillUpdateLastUsedTime(WebappDataStorage storage) {
+        super.onWillUpdateLastUsedTime(storage);
+
+        long lastUsedMs = storage.getLastUsedTime();
+        if (storage.wasLaunched()) {
+            WebApkUma.recordLaunchInterval(lastUsedMs - storage.getLastUsedTime());

[Xi Han, 2017/06/09 14:55:35]

I am confused here. Do you expect the |LastUsedTime| be changed between calling
of line 189 and line 191? Looks like it will record 0 to the launch interval
metrics.

[pkotwicz, 2017/06/12 20:34:46]

Thank you for catching this bug!

+        }
+    }
+
     /**
      * If we're showing a WebApk that's not with an expected package, it must be an
      * "Unbound WebApk" (crbug.com/714735) so show a notification that it's running in Chrome.