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

chrome/android/java/src/org/chromium/chrome/browser/metrics/WebApkUma.java

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.metrics;
 
 import org.chromium.base.metrics.RecordHistogram;
 import org.chromium.webapk.lib.common.WebApkConstants;
 
 import java.util.concurrent.TimeUnit;
@@ skipping 114 matching lines @@
                 "WebApk.Install.GooglePlayBindDuration", durationMs, TimeUnit.MILLISECONDS);
     }
 
     /** Records the current Shell APK version. */
     public static void recordShellApkVersion(int shellApkVersion, String packageName) {
         String name = packageName.startsWith(WebApkConstants.WEBAPK_PACKAGE_PREFIX)
                 ? "WebApk.ShellApkVersion.BrowserApk"
                 : "WebApk.ShellApkVersion.UnboundApk";
         RecordHistogram.recordSparseSlowlyHistogram(name, shellApkVersion);
     }
+
+    /**
+     * Recorded when a WebAPK is launched from the homescreen. Records the time elapsed since the
+     * previous WebAPK launch. Not recorded the first time that a WebAPK is launched.
+     */
+    public static void recordLaunchInterval(long intervalMs) {
+        RecordHistogram.recordCustomTimesHistogram("WebApk.LaunchInterval", intervalMs,
+                TimeUnit.HOURS.toMillis(1), TimeUnit.DAYS.toMillis(30), TimeUnit.MILLISECONDS, 50);
+    }
 }