Consolidate Font::codePath kerning & ligatures exceptions

Currently, we tweak the computed code path in several places to force
kerning & ligatures handling via the complex path. These checks
are identical and can be handled in codePath() instead.

The refactoring preserves existing logic.

R=eae@chromium.org,jbroman@chromium.org,dominik.rottsches@intel.com

Review URL: https://codereview.chromium.org/642863005

git-svn-id: svn://svn.chromium.org/blink/trunk@183562 bbb929c8-8fbe-4397-9dbb-9b2b20218538