Adds UMA for PrerenderingOffliner request processing result status.

components/offline_pages/background/offliner.h

 // 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 COMPONENTS_OFFLINE_PAGES_BACKGROUND_OFFLINER_H_
 #define COMPONENTS_OFFLINE_PAGES_BACKGROUND_OFFLINER_H_
 
 #include "base/callback.h"
 
 namespace offline_pages {
 
 class SavePageRequest;
 
 // Interface of a class responsible for constructing an offline page given
 // a request with a URL.
 class Offliner {
  public:
   // Status of processing an offline page request.
-  enum class RequestStatus {
-    UNKNOWN,      // No status determined/reported yet.
-    LOADED,       // Page loaded but not (yet) saved.
-    SAVED,        // Offline page snapshot saved.
-    CANCELED,     // Request was canceled.
-    FAILED,       // Failed to load page.
-    FAILED_SAVE,  // Failed to save loaded page.
-    // TODO(dougarnett): Define a retry-able failure status.
+  // WARNING: You must update histograms.xml to match any changes made to
+  // this enum (ie, OfflinePagesBackgroundOfflinerRequestStatus histogram enum).
+  enum RequestStatus {
+    UNKNOWN = 0,                       // No status determined/reported yet.
+    LOADED = 1,                        // Page loaded but not (yet) saved.
+    SAVED = 2,                         // Offline page snapshot saved.
+    REQUEST_COORDINATOR_CANCELED = 3,  // RequestCoordinator canceled request.
+    PRERENDERING_CANCELED = 4,         // Prerendering was canceled.
+    PRERENDERING_FAILED = 5,           // Prerendering failed to load page.
+    SAVE_FAILED = 6,                   // Failed to save loaded page.
+    // NOTE: insert new values above this line and update histogram enum too.
+    STATUS_COUNT
   };
 
   // Reports the completion status of a request.
   // TODO(dougarnett): consider passing back a request id instead of request.
   typedef base::Callback<void(const SavePageRequest&, RequestStatus)>
       CompletionCallback;
 
   Offliner() {}
   virtual ~Offliner() {}
 
   // Processes |request| to load and save an offline page.
   // Returns whether the request was accepted or not. |callback| is guaranteed
   // to be called if the request was accepted and |Cancel()| is not called.
   virtual bool LoadAndSave(
       const SavePageRequest& request,
       const CompletionCallback& callback) = 0;
 
-  // Clears the currently processing request, if any.
+  // Clears the currently processing request, if any, and skips running its
+  // CompletionCallback.
   virtual void Cancel() = 0;
 
   // TODO(dougarnett): add policy support methods.
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_BACKGROUND_OFFLINER_H_