Make style building for font-weight less custom.

This patch lets the StyleBuilderConverter produce the final values
for CSSValueBolder and CSSValueLighter, instead of first setting the
weight on FontBuilder, and then setting the weight again.

This is more in line with how the rest of the style building works.

Review URL: https://codereview.chromium.org/444413003

git-svn-id: svn://svn.chromium.org/blink/trunk@179755 bbb929c8-8fbe-4397-9dbb-9b2b20218538