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This object is responsible for combining all attached keyboards into one
logical keyboard, applying modifiers & implementing key repeat. Key
repeat is not done yet.

We need this so we can also dispatch keypresses from gestures devices
(e.g. Logitech K400).

BUG=412996
TEST=events_unittests, link_freon

Review URL: https://codereview.chromium.org/641113004

Cr-Commit-Position: refs/heads/master@{#300270}