Create skeleton for the Prefetching store and initial pipeline step.

components/offline_pages/core/prefetch/prefetch_dispatcher.h

Index: components/offline_pages/core/prefetch/prefetch_dispatcher.h
diff --git a/components/offline_pages/core/prefetch/prefetch_dispatcher.h b/components/offline_pages/core/prefetch/prefetch_dispatcher.h
index 36f9b206de3b4931cd2951798012fbeac27d9977..0e8850faf619c27b3c7d6c2ff4f560ddec3319cb 100644
--- a/components/offline_pages/core/prefetch/prefetch_dispatcher.h
+++ b/components/offline_pages/core/prefetch/prefetch_dispatcher.h
@@ -11,8 +11,7 @@
 #include "base/memory/weak_ptr.h"
 #include "components/offline_pages/core/offline_page_item.h"
 #include "components/offline_pages/core/prefetch/prefetch_dispatcher.h"
-
-class GURL;
+#include "components/offline_pages/core/prefetch/prefetch_types.h"
 
 namespace offline_pages {
 
@@ -21,16 +20,6 @@ namespace offline_pages {
 // and manage their execution and inter-dependencies.
 class PrefetchDispatcher {
  public:
-  struct PrefetchURL {
-    // Used to differentiate URLs from different sources.  |name_space| should
-    // be unique per source. |id| can be anything useful to identify the page,
-    // but will not be used for deduplication.
-    ClientId client_id;
-
-    // This URL will be prefetched by the system.
-    GURL url;
-  };
-
   // A |ScopedBackgroundTask| is created when we are running in a background
   // task.  Destroying this object should notify the system that we are done
   // processing the background task.
@@ -46,9 +35,11 @@ class PrefetchDispatcher {
 
   virtual ~PrefetchDispatcher() = default;
 
-  // Called when a consumer has candidate URLs for the system to prefetch.
-  // Duplicates are accepted by the PrefetchDispatcher but ignored.
+  // Called when a client has candidate URLs for the system to prefetch, along
+  // with the client's unique namespace. URLs that are currently in the system
+  // for this client are acceptable but ignored.
   virtual void AddCandidatePrefetchURLs(
+      const std::string& name_space,
       const std::vector<PrefetchURL>& prefetch_urls) = 0;
 
   // Called when all existing suggestions are no longer considered valid for a
@@ -59,7 +50,8 @@ class PrefetchDispatcher {
 
   // Called to invalidate a single PrefetchURL entry identified by |client_id|.
   // If multiple have the same |client_id|, they will all be removed.
-  virtual void RemovePrefetchURLsByClientId(const ClientId& client_id) = 0;
+  virtual void RemovePrefetchURLsByClientId(const std::string& name_space,
+                                            const std::string& client_id) = 0;
 
   // Called when Android OS has scheduled us for background work.  When
   // destroyed, |task| will call back and inform the OS that we are done work