[Variations] Bring variations_http_header_provider to variations component

components/variations/proto/chrome_experiments.proto

 // Copyright 2014 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.
 //
 // Summary of Chrome variations from experiments.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package metrics;
 
 message ChromeVariations {
   // A list of Chrome experiment variation IDs that are active.
   repeated int32 variation_id = 1;
 
   // A list of Chrome experiment variation IDs that trigger server side
   // behavior.
   repeated int32 trigger_variation_id = 3;
 }