[unibrow] -1 used by kNoPreviousCharacter and kEndOfInput isn't a valid Lead/Trail char. Drop superfluous check.

Change-Id: Ie6b8e61230e25ab0321d0672d71f64fd593714bf
Reviewed-on: https://chromium-review.googlesource.com/1109829
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53924}