Convert Extensions TCP & TLS Socket APIs to mojo sockets

This CL converts extensions TCP and TLS socket apis to network service's mojo
socket.

- Remove combined_socket_unittest.cc because the test cases no longer apply. I
tried to write equivalent ones in tcp_socket_unittest.cc and
tls_socket_unittest.cc
- Add a ContentUtilityClient implementation for extensions_browsertests, so we
can inject a NetworkServiceTestHelper to the utility process in which network
service runs.

Bug: 721401
Cq-Include-Trybots: luci.chromium.try:linux_chromium_dbg_ng;master.tryserver.chromium.linux:linux_mojo
Change-Id: I333a3021a5c66eb94618be6830ba0ff68c442eba
Reviewed-on: https://chromium-review.googlesource.com/1070434
Commit-Queue: Helen Li <xunjieli@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#563390}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 41066e83216ea19036dcf33cfbf35e18f77da9fe

check_gn_headers_whitelist.txt

@@ -51,7 +51,6 @@ chrome/browser/component_updater/component_installer_errors.h
 chrome/browser/download/download_file_icon_extractor.h
 chrome/browser/extensions/api/networking_cast_private/chrome_networking_cast_private_delegate.h
 chrome/browser/extensions/api/omnibox/omnibox_api_testbase.h
-chrome/browser/extensions/api/socket/mock_tcp_client_socket.h
 chrome/browser/mac/bluetooth_utility.h
 chrome/browser/media/router/mojo/media_route_provider_util_win.h
 chrome/browser/media/webrtc/desktop_media_list_ash.h