[KeyboardLock] KeyboardHook impl and WindowTreeHost integration

This CL introduces a Windows implementation of the KeyboardHook class
as well as a simple integration with the low-level window classes used
in Aura for Windows.

Changes to WindowTreeHostPlatform / PlatformWindow will be made in a
separate CL.  I will send out  that change once the method names and
platform hook scope questions have been answered.

The next CL will build on this work in the content layer:
https://chromium-review.googlesource.com/c/chromium/src/+/905507

BUG=680809

Change-Id: I8d35c895d80a58c64ff5b2bc119d5ee340790050
Reviewed-on: https://chromium-review.googlesource.com/900344
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539170}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 8bc3936..04f009d7 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -67,6 +67,7 @@
     "mus/window_tree_host_mus.h",
     "mus/window_tree_host_mus_delegate.h",
     "mus/window_tree_host_mus_init_params.h",
+    "scoped_keyboard_hook.h",
     "scoped_window_targeter.h",
     "window.h",
     "window_delegate.h",
@@ -126,6 +127,7 @@
     "mus/window_tree_client_delegate.cc",
     "mus/window_tree_host_mus.cc",
     "mus/window_tree_host_mus_init_params.cc",
+    "scoped_keyboard_hook.cc",
     "scoped_window_targeter.cc",
     "window.cc",
     "window_event_dispatcher.cc",