Make font-weight animatable.

We currently don't animate font-weight, even though (1) the spec calls for it, and (2) other
browsers (IE+FF) do.

The interpolator converts the font weights to equivalent real numbers, and performs
the interpolation on these values. The font weight is rounded to the nearest 100 when
the animated style is built.

BUG=289734
TEST=automated

Review URL: https://codereview.chromium.org/173953002

git-svn-id: svn://svn.chromium.org/blink/trunk@167784 bbb929c8-8fbe-4397-9dbb-9b2b20218538