Add discovery capability for Sharing Device

Bug: 1043148
Change-Id: Ic0fcfb5dc26b2cfc59acbd15580ff58aa23fe5bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007417
Commit-Queue: Alex Chau <alexchau@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732914}

components/sync/protocol/device_info_specifics.proto

@@ -101,6 +101,7 @@ message SharingSpecificFields {
     SMS_FETCHER = 3;
     REMOTE_COPY = 4;
     PEER_CONNECTION = 5;
+    DISCOVERY = 6;
   }
 
   // A list of enabled Sharing features.