Clean up CSSParser::parseValue interface and usage

Many of the callers to CSSParser::parseValue use it to convert a string
into a single CSSValue. This patch moves the logic for this into
CSSParser::parseSingleValue. The parseFontFaceValue logic has been moved
up into the only caller in CSSValuePool to use the added function.

I've removed the comment about unifying the parseValue functions in this
patch; it looks like one of them is an optimisation to avoid creating
a CSSParserContext if possible.

BUG=330389

Review URL: https://codereview.chromium.org/566253003

git-svn-id: svn://svn.chromium.org/blink/trunk@182169 bbb929c8-8fbe-4397-9dbb-9b2b20218538