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This CL adds a minimal KeyboardHook impl for X11 which enables
browser-level keyboard lock functionality.

I split up the original X11 KeyboardHook impl into two pieces as
the browser level keyboard lock functionality is well understood
and can be tested now whereas the system level keyboard lock for
X11 has a few questions that need to be resolved before checking
it in.

Therefore I want to get the the browser-level lock functionality
working so it can be tested in Canary while I sort out the
system shortcut aspect of the impl.

BUG=680809

Change-Id: I47ea23f50d6c61364343574ab4cdc73bd17f0c3f
Reviewed-on: https://chromium-review.googlesource.com/1034272Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554822}