Use floats in HarfBuzzShaper for word and letter spacing

Blink r153727 added support for fractional values for word and
letter spacing to Blink. These values were being truncated in
HarfBuzz's complex text path down to integers, leading to
layout issues. Changing the values to floats so text is properly
measured by HarfBuzzShaper.

BUG=336358

Review URL: https://codereview.chromium.org/140823014

git-svn-id: svn://svn.chromium.org/blink/trunk@165577 bbb929c8-8fbe-4397-9dbb-9b2b20218538