Update SharingMessage proto to remove unnecessary fields

- message_id is already provided by IncomingMessage
- sent_time is already provided by FCM, can be added to IncomingMessage

Bug: 966028
Change-Id: Ia6247f262659f0748ffaa4c81d61757dbf84c54b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636052
Auto-Submit: Alex Chau <alexchau@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665176}

chrome/browser/sharing/proto/sharing_message.proto

@@ -11,19 +11,13 @@ option optimize_for = LITE_RUNTIME;
 
 // Message for sending between devices in Sharing.
 message SharingMessage {
-  // Unique message identifier. required.
-  string message_id = 1;
-
   // Identifier of sender. required.
-  string sender_guid = 2;
-
-  // Timestamp at which message was sent. required.
-  int64 send_timestamp = 3;
+  string sender_guid = 1;
 
   // Payload of the message, contains one of the messages below. required.
   oneof payload {
-    PingMessage ping_message = 4;
-    AckMessage ack_message = 5;
+    PingMessage ping_message = 2;
+    AckMessage ack_message = 3;
   }
 }