[Sync::Proto] Add a comment that EncryptedData blob is base64 encoded.

This was not obvious on the server and led to a confusion.
Happy to provide more details in a private discussion.

Change-Id: I7214daf5ae0daa7e5213a921846cde0c5c49bf38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484693
Commit-Queue: vitaliii <vitaliii@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Auto-Submit: vitaliii <vitaliii@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818427}

components/sync/protocol/encryption.proto

@@ -30,5 +30,5 @@ package sync_pb;
 // node.
 message EncryptedData {
   optional string key_name = 1;
-  optional string blob = 2;
+  optional string blob = 2;  // base64-encoded.
 }