Remove FontCacheKey::setSynthetic()

Set these bits in FontDescription::cacheKey().
Only CSSSegmentedFontFace does not use these bits, but synthetic bold/italic
bits are always zero there.

TEST=none, no behavior changes
BUG=

Review URL: https://codereview.chromium.org/86963003

git-svn-id: svn://svn.chromium.org/blink/trunk@162665 bbb929c8-8fbe-4397-9dbb-9b2b20218538