[OnionSoup] typemap network.mojom.HTTPRequestHeaders with net::HttpRequestHeaders for Blink variant

This CL changes the typemapping of network.mojom.HttpRequestHeaders
for Blink variant. See details below:

Before: typemap with blink::HTTPHeaderMap
After : typemap with net::HttpRequestHeaders

This CL is part of a series of CLs which are targeted to typemap
network.mojom.URLRequest with network::ResourceRequest for Blink variant
finally.

1. https://chromium-review.googlesource.com/c/chromium/src/+/1669131
2. https://chromium-review.googlesource.com/c/chromium/src/+/1672768
3. This CL
4. https://chromium-review.googlesource.com/c/chromium/src/+/1679984
5. https://chromium-review.googlesource.com/c/chromium/src/+/1683493

BUG=860403

Change-Id: I29111a6ef8a094028c8f7919cca7761f9e1d84a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1680505
Commit-Queue: Minggang Wang <minggang.wang@intel.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676260}

third_party/blink/renderer/platform/mojo/blink_typemaps.gni

@@ -4,6 +4,7 @@
 
 typemaps = [
   "//ui//gfx//mojo//transform.typemap",
+  "//services/network/public/cpp/http_request_headers.typemap",
   "//third_party/blink/renderer/core/messaging/blink_cloneable_message.typemap",
   "//third_party/blink/renderer/core/messaging/blink_transferable_message.typemap",
   "//third_party/blink/renderer/modules/indexeddb/indexed_db_blink.typemap",
@@ -18,7 +19,6 @@ typemaps = [
   "//third_party/blink/renderer/platform/mojo/time.typemap",
   "//third_party/blink/renderer/platform/network/encoded_form_data_element.typemap",
   "//third_party/blink/renderer/platform/network/encoded_form_data.typemap",
-  "//third_party/blink/renderer/platform/network/http_request_headers.typemap",
   "//third_party/blink/public/common/manifest/display_mode.typemap",
   "//third_party/blink/public/common/screen_orientation/screen_orientation_lock_types.typemap",
   "//third_party/blink/public/mojom/bluetooth/bluetooth.typemap",

third_party/blink/renderer/platform/network/http_request_headers.typemap

-# Copyright 2019 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.
-
-mojom = "//services/network/public/mojom/http_request_headers.mojom"
-public_headers =
-    [ "//third_party/blink/renderer/platform/network/http_header_map.h" ]
-traits_headers = [ "//third_party/blink/renderer/platform/network/http_request_headers_mojom_traits.h" ]
-type_mappings = [ "network.mojom.HttpRequestHeaders=::blink::HTTPHeaderMap" ]