[KeyboardLock] Adding a SystemKeyboardLock feature

This feature controls whether the low-level keyboard hook is enabled
when keyboard lock is requested.  I wanted to separate browser and
system level keyboard lock functionality as I get closer to launching
the blink api for keyboard lock.  This will allow us to release
browser-level keyboard lock w/o the system hooks if needed and will
also give us the ability to target a finch kill switch for the
system-level functionality if needed.

BUG=680809

Change-Id: I2d522046c90fd8776bfb1d9c53978f2c41e1c7cc
Reviewed-on: https://chromium-review.googlesource.com/1046154
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558367}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 0a6f3bf7..c5eb719e 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -70,6 +70,7 @@
     "mus/window_tree_host_mus_delegate.h",
     "mus/window_tree_host_mus_init_params.h",
     "scoped_keyboard_hook.h",
+    "scoped_simple_keyboard_hook.h",
     "scoped_window_targeter.h",
     "window.h",
     "window_delegate.h",
@@ -134,6 +135,7 @@
     "mus/window_tree_host_mus.cc",
     "mus/window_tree_host_mus_init_params.cc",
     "scoped_keyboard_hook.cc",
+    "scoped_simple_keyboard_hook.cc",
     "scoped_window_targeter.cc",
     "window.cc",
     "window_event_dispatcher.cc",