Defined constants for all 9 CSS font-weight values
with conventional names from the "Common weight
name mapping" section of
https://developer.mozilla.org/en-US/docs/Web/CSS/font-weight.

The FontWeight enum now just enumerates the actual
CSS weight values.

I've moved the TextStyle class into its own file.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1173323004.