Only allocate the hyphen-appending buffer if we intend to use it.

Otherwise, there is no apparent need to reserve this capacity. This
reduces time spent painting text.

BUG=270678
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/22745002

git-svn-id: svn://svn.chromium.org/blink/trunk@155848 bbb929c8-8fbe-4397-9dbb-9b2b20218538