Reland "Serialize 'oblique' computed values for 'font-style'"

Handling of 'oblique' values were missing in
ComputedStyleUtils::ValueForFontStyle - serialize as 'oblique <angle>'.
Because of this the return value needs to be widened to CSSValue.

Expectations for css/css-fonts/variations/font-shorthand.html updated.

TBR=drott@chromium.org

Bug: 809935, 809938
Change-Id: I753324776ba080820bc5c4a14e8aff6237ee5d12
Reviewed-on: https://chromium-review.googlesource.com/995793
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#548143}
4 files changed