Add taskid for navigation, created in session sync

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 0c8095d2233ef791c9627d4a028e3d5c3e8a4b8a..77acb2519ebe2b3fb5011f003aeef8e4ac290323 100644
--- a/components/sync/protocol/session_specifics.proto
+++ b/components/sync/protocol/session_specifics.proto
@@ -156,6 +156,30 @@ message TabNavigation {
   }
   // Whether the Password Manager saw a password field on the page.
   optional PasswordState password_state = 26;
+
+  // Id of the task associated with this navigation.
+  optional TaskId task_id = 27;
+}
+
+// Task id which recursively encodes id of its parent task. From one task id,
+// we can get ids of its all ancester tasks.

[Nicolas Zea, 2017/03/27 20:43:52]

Explain what a task node id is (namely a GUID that defines a specific navigation
that matches global_id above)

[shenchao, 2017/04/06 00:54:55]

Done.

+// E.g. if current task id is
+// {
+//   task_node_id:1
+//   task_node_id:2
+//   task_node_id:3
+// }, then id of its parent task is
+// {
+//   task_node_id:1
+//   task_node_id:2
+// }, and id of its grandparent task, which is a root task is
+// {
+//   task_node_id:1
+// }
+message TaskId {
+  // A list of node ids, not encoding parent node, for task path of (root,
+  // ..., parent, current task).
+  repeated int64 task_node_id = 1;

[Nicolas Zea, 2017/03/27 20:43:52]

Should we bother encoding the current task id/ It's basically going to be the
same as the global id above right?

[shenchao, 2017/04/04 18:37:50]

Just found we can't use global_id, which changes when the page is visited via
forward_back. But we want to keep task_node_id same for different visits over
time, right?

 }
 
 // Navigation information for a single redirection within a single navigation.