Simplify StyleBuilder handling of font-stretch

The generated code basically does what we want for font-stretch, so we
should use it instead of defining custom handlers.

BUG=374530

Review URL: https://codereview.chromium.org/432523003

git-svn-id: svn://svn.chromium.org/blink/trunk@179286 bbb929c8-8fbe-4397-9dbb-9b2b20218538