Prefer synthetic bold/italic over alternate family

Change the font fallback code to prefer the specified font family with
syntetic bold and/or italic over falling back to an alternate font.

R=pdr@chromium.org
BUG=385069
TEST=fast/text/font-fallback-synthetic-italics.html

Review URL: https://codereview.chromium.org/361833003

git-svn-id: svn://svn.chromium.org/blink/trunk@177327 bbb929c8-8fbe-4397-9dbb-9b2b20218538