match comment changes from CL on server side

Bug: 901429
Change-Id: I888c28fb2900e2c780c654ebb2a92450ec9a6c59
Reviewed-on: https://chromium-review.googlesource.com/c/1331578Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Jeffrey Cohen <jeffreycohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607637}

components/sync/protocol/send_tab_to_self_specifics.proto

@@ -15,22 +15,23 @@ package sync_pb;
 
 // Send Tab To Self list entry. This proto contains the fields synced to send
 // a url across devices.
-
 message SendTabToSelfSpecifics {
   // The name of the tab being shared.
   optional string title = 1;
   // The URL of the tab being shared.
+  // Required
   optional string url = 2;
   // The time the tab was shared as measured by the client in microseconds since
   // the windows epoch.
+  // Required
   optional int64 shared_time_usec = 3;
   // The device where this tab originated.
   optional SendTabToSelfDevice from_device = 4;
 }
 
 message SendTabToSelfDevice {
-  // A non-unique but human readable name to describe this client.
+  // A non-unique but human readable name to describe this client, used in UI.
   optional string name = 1;
-  // Device_id that is stable until user signs out.
+  // Device_id that is stable until user signs out to avoid round tripping.
   optional string signin_scoped_device_id = 2;
 }