Extend WidthCache to include glyph overflow bounds

Extend the font WidthCache to always compute and cache the glyph bounding box,
regardless of whether the glyphOverflow parameter is supplied. This ensures that
subsequent calls can always be served from the cache and as such the cache can
now also be used for text measurements where the bounding box of the glyphs is
required. 

BUG=347186
R=dominik.rottsches@intel.com

Review URL: https://codereview.chromium.org/285343002

git-svn-id: svn://svn.chromium.org/blink/trunk@174230 bbb929c8-8fbe-4397-9dbb-9b2b20218538