Add plumbing for font-stretch

Implement support for parsing and querying the font-stretch CSS
property, for extracting the font-stretch from the short-hand font
property, and for querying the font-stretch through getComputedStyle.

Note that this patch does _not_ implement the font-stretch
functionality itself, it merely adds the required plumbing.

Request to implement thread:
  https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/N24I2zJemvI

Entry in Chromium Dashboard:
  http://www.chromestatus.com/features/4598830058176512

R=dglazkov@chromium.org
BUG=331119
TEST=fast/css3-text/font-stretch.html

Review URL: https://codereview.chromium.org/199423003

git-svn-id: svn://svn.chromium.org/blink/trunk@169264 bbb929c8-8fbe-4397-9dbb-9b2b20218538