Implement an observer interface for the site engagement service.

chrome/browser/engagement/site_engagement_observer.h

Index: chrome/browser/engagement/site_engagement_observer.h
diff --git a/chrome/browser/engagement/site_engagement_observer.h b/chrome/browser/engagement/site_engagement_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..5ecc4214e5b907c24c9a445e61a97792c4e6f5ef
--- /dev/null
+++ b/chrome/browser/engagement/site_engagement_observer.h
@@ -0,0 +1,42 @@
+// Copyright 2016 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.
+
+#ifndef CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_
+#define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_
+
+#include "base/gtest_prod_util.h"
+#include "base/macros.h"
+
+class GURL;
+class SiteEngagementService;
+
+class SiteEngagementObserver {
+ public:
+  // Called when the engagement for |url| increases to |score|.
+  // This may be run on user input, so observers *must not* perform any
+  // expensive tasks in this callback.
+  virtual void OnEngagementIncreased(const SiteEngagementService* service,
+                                     const GURL& url,

[benwells, 2016/06/01 01:44:45]

can you use a url::Origin here instead?

There is a separate effort going on to use that class instead of GURL when the
urls in question should be origins.

[dominickn, 2016/06/01 02:54:13]

Do you think that the observer should only be informed of the origin for which
engagement increased, or the full URL which triggered that increase?

For instance, for app banners, the particular PWA may be hosted under a path,
and we want an engagement increase on that precise path before triggering the
banner flow. If we reduce this parameter to just an origin, we'll lose that
precision. Just a strawman, but I'd like to hear your thoughts.

[benwells, 2016/06/01 03:35:50]

Hmm ... I hadn't considered that. It feels a bit strange to limit it to paths,
as the engagement is for the origin as a whole, not any individual path.

In the app banner case I'd be worried about subtle race conditions and other
weirdness occuring, but it's hard to consider it without seeing code.

Either way, feel free to leave it as is if you'll be passing the full url
through, and change to an Origin if it will just be origins.

+                                     double score) {}
+
+ protected:
+  explicit SiteEngagementObserver(SiteEngagementService* service);
+
+  SiteEngagementObserver();
+
+  ~SiteEngagementObserver();
+
+  // Begin observing |service| for engagement increases.
+  // To stop observing, call Observe(nullptr).
+  void Observe(SiteEngagementService* service);
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, Observers);
+  SiteEngagementService* service_;
+
+  DISALLOW_COPY_AND_ASSIGN(SiteEngagementObserver);
+
+};
+
+#endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_