The CharacterRange constructor checks the input for validity. However,
CharacterRange::Singleton also uses the constructor and may have
kEndMarker as input, causing the check to fail.

The solution is to move the check to CharacterRange::Range and
consistently use it across the code base.

R=jkummerow@chromium.org
BUG=chromium:593282
LOG=N

Review URL: https://codereview.chromium.org/1776013003

Cr-Commit-Position: refs/heads/master@{#34626}