remoting: Initial Linux keyboard layout monitor implementation.

This still has a couple of rough edges:
 * Some layouts lacking a keypad thousands separator will show a decimal
   separator at both NumpadDecimal and NumpadComma.
 * IntlBackslash will appear on layouts where it's not typically
   present (usually generating '<').
 * Japanese and Korean IME keys are not included even where applicable.
 * When the user modifies the enabled keyboard layouts or settings,
   several layout messages can end up being sent in rapid succession as
   the changes are applied.
 * Probably other things that will be revealed during testing.

Bug: 1026029
Change-Id: Icb511db3d3b5715ab563a0f433443c977e5c805c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063850
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744150}

remoting/host/BUILD.gn

@@ -232,6 +232,7 @@ static_library("common") {
     "it2me_desktop_environment.h",
     "keyboard_layout_monitor.cc",
     "keyboard_layout_monitor.h",
+    "keyboard_layout_monitor_chromeos.cc",
     "keyboard_layout_monitor_linux.cc",
     "keyboard_layout_monitor_mac.cc",
     "keyboard_layout_monitor_win.cc",
@@ -393,6 +394,7 @@ static_library("common") {
       "continue_window_linux.cc",
       "curtain_mode_linux.cc",
       "disconnect_window_linux.cc",
+      "keyboard_layout_monitor_linux.cc",
       "me2me_desktop_environment.cc",
       "me2me_desktop_environment.h",
       "xsession_chooser_linux.cc",

remoting/host/keyboard_layout_monitor_chromeos.cc

+// Copyright 2020 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.
+
+#include "remoting/host/keyboard_layout_monitor.h"
+
+#include "base/callback.h"
+
+namespace remoting {
+
+namespace {
+
+class KeyboardLayoutMonitorChromeOs : public KeyboardLayoutMonitor {
+ public:
+  explicit KeyboardLayoutMonitorChromeOs(
+      base::RepeatingCallback<void(const protocol::KeyboardLayout&)> callback) {
+  }
+  ~KeyboardLayoutMonitorChromeOs() override = default;
+  void Start() override { NOTIMPLEMENTED(); }
+};
+
+}  // namespace
+
+std::unique_ptr<KeyboardLayoutMonitor> KeyboardLayoutMonitor::Create(
+    base::RepeatingCallback<void(const protocol::KeyboardLayout&)> callback,
+    scoped_refptr<base::SingleThreadTaskRunner> input_task_runner) {
+  return std::make_unique<KeyboardLayoutMonitorChromeOs>(std::move(callback));
+}
+
+}  // namespace remoting