Allow hex-style colors to be parsed by fastParseColor in strict mode

Hex-style colors (#rrggbb, #rgb) are valid in strict mode too so allow
them to be parsed if |strict| == true.
This prevents unnecessarily falling back to the Bison parser in
BisonCSSParser::parseColor in these cases.

BUG=365560

Review URL: https://codereview.chromium.org/246833005

git-svn-id: svn://svn.chromium.org/blink/trunk@172150 bbb929c8-8fbe-4397-9dbb-9b2b20218538