Replace FontTraitsMask with struct

Replace the FontTraitsMask bit fiddling with a struct and change
FontDescription, FontFace, CSSSegmentedFontFace and FontFaceCache to
use the struct instead of the mask thereby greatly simplifying the
code.

BUG=351734
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/197803003

git-svn-id: svn://svn.chromium.org/blink/trunk@169064 bbb929c8-8fbe-4397-9dbb-9b2b20218538