[Sync] Enable Compression from Client to Server by experiment

components/sync/protocol/sync.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Sync protocol for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 413 matching lines @@
 
   // The configuration of this client at commit time. Used by the server to
   // make commit-time decisions about how to process datatypes that might
   // involve server-side interaction, and e.g require explicit user intent for
   // syncing a particular data type regardless of whether a commit for that
   // datatype is currently being sent up.
   optional ClientConfigParams config_params = 4;
 
   // Set of optional per-client datatype contexts.
   repeated DataTypeContext client_contexts = 5;
+
+  // This field need to be 256 bytes if set. This attempts to mitigate CRIME
+  // attacks when sync communicate from client to server.

[Nicolas Zea, 2016/10/18 20:23:08]

Call out that this is a 256 byte random string.

Also mention that this is explicitly for clients with compression of client to
server data enabled. If no compression is used, this should not be set.

[Gang Wu, 2016/10/19 00:06:14]

Done.

+  optional string padding = 6;
 };
 
 // This message communicates additional per-type information related to
 // requests with origin GU_TRIGGER.  This message is not relevant when any
 // other origin value is used.
 // Introduced in M29.
 message GetUpdateTriggers {
   // An opaque-to-the-client string of bytes, received through a notification,
   // that the server may interpret as a hint about the location of the latest
   // version of the data for this type.
@@ skipping 587 matching lines @@
 message EventResponse {};
 
 // A message indicating that the sync engine has been disabled on a client.
 message SyncDisabledEvent {
   // The GUID that identifies the sync client.
   optional string cache_guid = 1;
 
   // The store birthday that the client was using before disabling sync.
   optional string store_birthday = 2;
 };