Pull fast-path parsing logic in BisonCSSParser-in.cpp into a new class CSSParserFastPaths

This patch is about making BisonCSSParser-in.cpp only contain code for
interacting with Bison. This patch pulls out the fast-path parsing logic
into a new class CSSParserFastPaths. A subsequent patch will move this
into the correct file (https://codereview.chromium.org/622493002/), but
this is left where it is for now to make reviewing easier.

I've changed the fast-path logic to simply return a CSSValue instead of
modifying a StylePropertySet, as this gives us a saner interface. The
CSSParser now only calls a single BisonCSSParser::parseValue function,
from a single location.

BUG=419270

Review URL: https://codereview.chromium.org/617863002

git-svn-id: svn://svn.chromium.org/blink/trunk@183377 bbb929c8-8fbe-4397-9dbb-9b2b20218538