[TextAutosizer] Make it clear that maxClusterWidthProvider is logically const

This patch makes maxClusterWidthProvider const which is not obvious from
a quick scan of the code. As broken as const is in C++, I hope this will
help folks understand this area in the future.

R=skobes

Review URL: https://codereview.chromium.org/595683004

git-svn-id: svn://svn.chromium.org/blink/trunk@182518 bbb929c8-8fbe-4397-9dbb-9b2b20218538