[Sync] Add support for identifying tabbed activites

components/sync/protocol/session_specifics.proto

Index: components/sync/protocol/session_specifics.proto
diff --git a/components/sync/protocol/session_specifics.proto b/components/sync/protocol/session_specifics.proto
index 8c9ebfb37d4850e59d20d6b90a2979f9e19d3dca..cedd4723dac6e83704b1b11a598be24ca7ba80cb 100644
--- a/components/sync/protocol/session_specifics.proto
+++ b/components/sync/protocol/session_specifics.proto
@@ -40,11 +40,12 @@ message SessionWindow {
   optional int32 window_id = 1;
   // Index of the selected tab in tabs; -1 if no tab is selected.
   optional int32 selected_tab_index = 2 [default = -1];
-  // Type of the browser. Currently we only store browsers of type
-  // TYPE_TABBED and TYPE_POPUP.
+  // Type of the window.
   enum BrowserType {
-    TYPE_TABBED = 1;
-    TYPE_POPUP = 2;
+    TYPE_TABBED = 1;      // A normal tabbed window.
+    TYPE_POPUP = 2;       // A popup window.
+    TYPE_CUSTOM_TAB = 3;  // A custom tab within an app. Not restored
+                          // automatically on sync startup.
   }
   optional BrowserType browser_type = 3 [default = TYPE_TABBED];
   // The tabs that compose a window (correspond to tab id's).