Refactor the offline page storage to include client namespace and id.

components/offline_pages/proto/offline_pages.proto

Index: components/offline_pages/proto/offline_pages.proto
diff --git a/components/offline_pages/proto/offline_pages.proto b/components/offline_pages/proto/offline_pages.proto
index aeab8b0c193ebf7f2cf64fa4ead9240dcbf31f5d..f87e066afc0a20f2e644c63fd51ca772e928926f 100644
--- a/components/offline_pages/proto/offline_pages.proto
+++ b/components/offline_pages/proto/offline_pages.proto
@@ -14,16 +14,19 @@ package offline_pages;
 
 message OfflinePageEntry {
   // URL of the offline page.
-  required string url = 1;
+  optional string url = 1;
 
   // Bookmark ID of the offline page.
-  required int64 bookmark_id = 2;
+  optional int64 deprecated_bookmark_id = 2;
+
+  // Offline ID of the page.  Offline ID has replaced Bookmark ID as the primary key
+  optional int64 offline_id = 10;
 
   // Version of the offline page metadata.
-  required int32 version = 3;
+  optional int32 version = 3;
 
   // Path to the offline archive.
-  required string file_path = 4;
+  optional string file_path = 4;
 
   // Size of the offline archive.
   optional int64 file_size = 5;
@@ -48,4 +51,9 @@ message OfflinePageEntry {
 
   // Flags for the offline page.
   optional Flags flags = 9;
+
+  // Information about this offline page in the namespace/id of the client that
+  // requested that it be saved. Useful for reverse lookups.
+  optional string client_id_namespace = 11;
+  optional string client_id = 12;
 }