Rename KEY_ constants to avoid conflict with <linux/input.h>

Most |enum class DomCode| constants are named by mapping the names from
the W3C specification from CamelCase to SHOUTY_CASE. For the Latin
letters, named 'KeyX' in the spec, the KEY_X form conflicts with macro
definitions in <linux/input.h>. This change renames DomCode::KEY_X
constants to DomCode::US_X to avoid the conflict and to highlight
that these are positional codes corresponding to the labels
on a US QWERTY layout.

(TBR for 15 files with only spelling changes.)

R=wez@chromium.org
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/1560293002

Cr-Commit-Position: refs/heads/master@{#368075}
24 files changed