Convert some component source sets to static library

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.
 
-source_set("net") {
+static_library("net") {
   sources = [
     "variations_http_headers.cc",
     "variations_http_headers.h",
   ]
 
   public_deps = [
     "//components/variations",
     "//net",
     "//url",
   ]
   deps = [
     "//base",
     "//components/google/core/browser",
     "//components/metrics",
     "//components/variations/proto",
   ]
 }