Add UKM API GetNewSourceID()

components/ukm/ukm_service.h

 // 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.
 
 #ifndef COMPONENTS_UKM_UKM_SERVICE_H_
 #define COMPONENTS_UKM_UKM_SERVICE_H_
 
 #include <stddef.h>
 #include <memory>
 #include <vector>
@@ skipping 31 matching lines @@
 // uploading reports that contain fine grained performance metrics including
 // URLs for top-level navigations.
 class UkmService : public base::SupportsWeakPtr<UkmService> {
  public:
   // Constructs a UkmService.
   // Calling code is responsible for ensuring that the lifetime of
   // |pref_service| is longer than the lifetime of 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);
 
   // Initializes the UKM service.
   void Initialize();
 
   // Enables/disables recording control if data is allowed to be collected.
   void EnableRecording();
   void DisableRecording();
@@ skipping 114 matching lines @@
   // Weak pointers factory used to post task on different threads. All weak
   // pointers managed by this factory have the same lifetime as UkmService.
   base::WeakPtrFactory<UkmService> self_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UkmService);
 };
 
 }  // namespace ukm
 
 #endif  // COMPONENTS_UKM_UKM_SERVICE_H_