Don't populate GlyphBuffer in InlineTextBox::characterWidths

The current InlineTextBox::characterWidths implementation populates a
GlyphBuffer even though it is never used. Change the implementation to
not pass a GlyphBuffer and thereby avoiding the unnecessary work and
allocation costs.

R=dominik.rottsches@intel.com

Review URL: https://codereview.chromium.org/608083003

git-svn-id: svn://svn.chromium.org/blink/trunk@182843 bbb929c8-8fbe-4397-9dbb-9b2b20218538