Add UKM API GetNewSourceID()

components/ukm/ukm_service.h

Index: components/ukm/ukm_service.h
diff --git a/components/ukm/ukm_service.h b/components/ukm/ukm_service.h
index 1775d16a1b7fb69218939aeb0f1686f9e12e7306..03e3ac3c00bf2788ec298f5450ba37f357f6ee32 100644
--- a/components/ukm/ukm_service.h
+++ b/components/ukm/ukm_service.h
@@ -49,6 +49,9 @@ class UkmService : public base::SupportsWeakPtr<UkmService> {
   UkmService(PrefService* pref_service, metrics::MetricsServiceClient* client);
   virtual ~UkmService();
 
+  // Get the new source ID.

[Bryan McQuade, 2017/03/01 22:46:35]

let's expand this comment to say something like 'Each returned ID is guaranteed
to be unique for the duration of a browser session.'

[Zhen Wang, 2017/03/01 22:51:42]

Done.

+  static int32_t GetNewSourceID();

[Zhen Wang, 2017/03/01 22:44:24]

We probably do not need static here. The benefit of using static is that even if
we have more than one UkmService instance, we still get unique new ID. But that
may not ever happen. Let me know if anyone would prefer to remove static here.

[Bryan McQuade, 2017/03/01 22:46:35]

I think static is totally fine - I'd use static here as well.

+
   // Update the URL on the source keyed to the given source ID. If the source
   // does not exist, it will create a new UkmSource object.
   void UpdateSourceURL(int32_t source_id, const GURL& url);