[Offline Pages] Adds event logs for requests dropped due to number of start or complete attempts.

components/offline_pages/background/offliner.h

Index: components/offline_pages/background/offliner.h
diff --git a/components/offline_pages/background/offliner.h b/components/offline_pages/background/offliner.h
index daa6d1893d2fb1609e1921ed17d7621a0ab889bc..5867d1e3d7a49912bc4515c59d2a22f96fc8f873 100644
--- a/components/offline_pages/background/offliner.h
+++ b/components/offline_pages/background/offliner.h
@@ -20,6 +20,7 @@ class Offliner {
   // Status of processing an offline page request.
   // WARNING: You must update histograms.xml to match any changes made to
   // this enum (ie, OfflinePagesBackgroundOfflinerRequestStatus histogram enum).
+  // Also update related switch code in RequestCoordinatorEventLogger.
   enum RequestStatus {
     // No status determined/reported yet. Interim status, not sent in callback.
     UNKNOWN = 0,