arc: Support pinned apps across Arc-enabled and Arc-disabled platforms.

sync/protocol/app_list_specifics.proto

Index: sync/protocol/app_list_specifics.proto
diff --git a/sync/protocol/app_list_specifics.proto b/sync/protocol/app_list_specifics.proto
index e18bc956c8e70614b8387f1512bb94986e7ed961..ed8f6be9cbb6888b7b5e7155ee8757e982e9bcc0 100644
--- a/sync/protocol/app_list_specifics.proto
+++ b/sync/protocol/app_list_specifics.proto
@@ -47,4 +47,10 @@ message AppListSpecifics {
 
   // Where on a page this item will appear.
   optional string item_ordinal = 6;
+
+  // Where on a shelf this item will appear.
+  optional string item_pin_ordinal = 7;
+
+  // If item is pinned by policy.
+  optional bool item_pin_by_policy = 8;
 }