[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}

• headless/lib/browser/headless_window_tree_host.cc[diff]
• headless/lib/browser/headless_window_tree_host.h[diff]
• ui/aura/BUILD.gn[diff]
• ui/aura/mus/window_tree_client_unittest.cc[diff]
• ui/aura/scoped_keyboard_hook.cc[Added - diff]
• ui/aura/scoped_keyboard_hook.h[Added - diff]
• ui/aura/window.cc[diff]
• ui/aura/window.h[diff]
• ui/aura/window_tree_host.cc[diff]
• ui/aura/window_tree_host.h[diff]
• ui/aura/window_tree_host_platform.cc[diff]
• ui/aura/window_tree_host_platform.h[diff]
• ui/events/BUILD.gn[diff]
• ui/events/event_constants.h[diff]
• ui/events/keyboard_hook.h[Added - diff]
• ui/events/keyboard_hook_base.cc[Added - diff]
• ui/events/keyboard_hook_base.h[Added - diff]
• ui/events/win/keyboard_hook_win.cc[Added - diff]
• ui/events/x/keyboard_hook_posix.cc[Added - diff]
• ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc[diff]
• ui/views/widget/desktop_aura/desktop_window_tree_host_win.h[diff]
• ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc[diff]
• ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h[diff]