Bug 1573238 - Provide a minimal BrowserUsageTelemetry.jsm module in GeckoView that implements nsIBrowserUsage; r=snorp

Differential Revision: https://phabricator.services.mozilla.com/D45428

--HG--
extra : moz-landing-system : lando

mobile/android/modules/geckoview/BrowserUsageTelemetry.jsm

@@ -0,0 +1,27 @@
+/* -*- js-indent-level: 2; indent-tabs-mode: nil -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+var EXPORTED_SYMBOLS = [
+  "getUniqueDomainsVisitedInPast24Hours",
+];
+
+// Used by nsIBrowserUsage
+function getUniqueDomainsVisitedInPast24Hours() {
+  // The prompting heuristic for the storage access API looks at 1% of the
+  // number of the domains visited in the past 24 hours, with a minimum cap of
+  // 5 domains, in order to prevent prompts from showing up before a tracker is
+  // about to obtain tracking power over a significant portion of the user's
+  // cross-site browsing activity (that is, we do not want to allow automatic
+  // access grants over 1% of the domains). We have the
+  // dom.storage_access.max_concurrent_auto_grants which establishes the
+  // minimum cap here (set to 5 by default) so if we return 0 here the minimum
+  // cap would always take effect. That would only become inaccurate if the
+  // user has browsed more than 500 top-level eTLD's in the past 24 hours,
+  // which should be a very unlikely scenario on mobile anyway.
+
+  return 0;
+}

mobile/android/modules/geckoview/moz.build

@@ -6,6 +6,7 @@
 
 EXTRA_JS_MODULES += [
     'AndroidLog.jsm',
+    'BrowserUsageTelemetry.jsm',
     'ContentCrashHandler.jsm',
     'DelayedInit.jsm',
     'GeckoViewAccessibility.jsm',