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- Add new DOM |code| entries corresponding to the USB HID
  codes for keyboard brightness. See USB HID spec update [1]
- Updating the US English keyboard layout data to map
  codes to existing keyboard-brightness accelerator VKEYS.

[1] - https://www.usb.org/sites/default/files/hutrr73_-_fn_key_and_keyboard_backlight_brightness_0.pdf

Bug=1076241
TEST=manual

Change-Id: I79573145afbc8bdafbc6da6782cd99099c73bc13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202667Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770685}