Sync: Add comments on removal of experiments protos

This adds comments to experiment_status.proto and
experiments_specifics.proto explaining why these deprecated definitions
can't be removed yet.

Bug: 1009361
Change-Id: Id2519a51d9c66a569015be7a58580b2299fe852b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032111
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Tim Schumann <tschumann@chromium.org>
Commit-Queue: Tim Schumann <tschumann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737294}

components/sync/protocol/experiment_status.proto

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file
 
+// NOTE: This API is not used in Chromium anymore, but the server still needs to
+// support it for the benefit of older clients (Chrome versions up to and
+// including M80 rely on it, see crbug.com/1009361).
+
 syntax = "proto2";
 
 option java_multiple_files = true;

components/sync/protocol/experiments_specifics.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 datatype extension for experimental feature flags.
 
-// If you change or add any fields in this file, update proto_visitors.h and
-// potentially proto_enum_conversions.{h, cc}.
+// This was a sync protocol datatype extension for experimental feature flags,
+// also exposed via a separate ExperimentStatus API. As of M75, the datatype
+// isn't used anymore, and as of M81 the ExperimentStatus API isn't used anymore
+// either (see crbug.com/939819 and crbug.com/1009361). The proto definition
+// needs to stay around for now so that the server can continue supporting these
+// old clients .
 
 syntax = "proto2";