[Clean up] Move VariationsHttpHeaderProvider into a subdirectory.

components/variations/net/BUILD.gn

 # 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.
 
-static_library("url_fixer") {
+source_set("net") {
   sources = [
-    "url_fixer.cc",
-    "url_fixer.h",
+    "variations_http_header_provider.cc",
+    "variations_http_header_provider.h",
   ]
 
+  public_deps = [ "//components/variations" ]
   deps = [
     "//base",
+    "//components/google/core/browser",
+    "//components/variations/proto",
     "//net",
   ]
-
-  if (is_win) {
-    # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
-    cflags = [ "/wd4267" ]
-  }
 }