ime: Change type of closed-source proto fields from string to bytes.

Wrapper.internal_request and Wrapper.internal_reply are the serialized
bytes of closed-source protos. However, string is not a suitable type,
as proto strings are designed for human readable strings and not bytes.
The right type for this is actually 'bytes' as it represents binary data
and not UTF-8 encoded human readable strings.

[1] https://developers.google.com/protocol-buffers/docs/proto#scalar

Bug: 1019541
Change-Id: I723c843acfb80eb14d63f7375cba1ddaf39a782c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397971Reviewed-by: Keith Lee <keithlee@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805650}

chromeos/services/ime/public/proto/messages.proto

@@ -7,8 +7,8 @@ package chromeos.ime;
 // Wrapper message to contain either internal messages or public messages.
 message Wrapper {
   oneof param {
-    string internal_request = 1;
-    string internal_reply = 2;
+    bytes internal_request = 1;
+    bytes internal_reply = 2;
     Request request = 3;
     Reply reply = 4;
   }