[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}

• chrome/browser/about_flags.cc[diff]
• chrome/browser/flag_descriptions.cc[diff]
• chrome/browser/flag_descriptions.h[diff]
• tools/metrics/histograms/enums.xml[diff]
• ui/aura/BUILD.gn[diff]
• ui/aura/scoped_simple_keyboard_hook.cc[Added - diff]
• ui/aura/scoped_simple_keyboard_hook.h[Added - diff]
• ui/aura/window_tree_host.cc[diff]
• ui/base/ui_base_features.cc[diff]
• ui/base/ui_base_features.h[diff]
• ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc[diff]
• ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc[diff]