Change text code to not use SimpleShaper members directly

Change InlineTextBox and Fonts to use the appropriate getters instead
of accessing member fields directly. Part of the effort to clean up
the SimpleShaper API.

BUG=418570
R=dominik.rottsches@intel.com

Review URL: https://codereview.chromium.org/608253003

git-svn-id: svn://svn.chromium.org/blink/trunk@182862 bbb929c8-8fbe-4397-9dbb-9b2b20218538