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 17d46c2d73d356dd80cd0e59c370455da8e15da2..a2734ab285f23939f2b12c082474d47fb104de7d 100644
--- a/components/offline_pages/proto/offline_pages.proto
+++ b/components/offline_pages/proto/offline_pages.proto
@@ -17,7 +17,11 @@ message OfflinePageEntry {
   optional string url = 1;
 
   // Bookmark ID of the offline page.

[dewittj, 2016/02/26 18:42:56]

nit: Change this comment to refer to legacy behavior rather than seemingly
current behavior.

[bburns, 2016/02/27 00:48:19]

Done.

-  optional int64 bookmark_id = 2;
+  optional int64 deprecated_bookmark_id = 2 [default = -1];
+
+  // Offline ID of the page.  Offline ID has replaced Bookmark ID as the primary

[dewittj, 2016/02/26 18:42:56]

Primary key is kinda undefined in this context.  Perhaps it's just better to say
that every OfflinePageEntry has a unique offline_id?

[bburns, 2016/02/27 00:48:19]

Done.

+  // key
+  optional int64 offline_id = 10;
 
   // Version of the offline page metadata.
   optional int32 version = 3;
@@ -48,4 +52,9 @@ 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_name_space = 11;
+  optional string client_id = 12;

[dewittj, 2016/02/26 18:42:56]

Please document that this is provided by consumers of offline pages.

[bburns, 2016/02/27 00:48:19]

Done.

 }