Remove custom style building for -webkit-font-feature-settings.

This patch replaces the custom style application for
-webkit-font-feature-settings with a function in StyleBuilderConverter.

Also:

 * Less use of CSSValue in FontBuilder.
 * We now handle 'initial' and 'inherit'.
 * Eliminates FontDescription::makeNormalFeatureSettings.

Note that in StyleBuilderConverter::convertFontFeatureSettings, the check
for isFontFeatureValue() is removed. Asserting (via toCSSFontFeatureValue)
should be enough, since the parser will only add CSSFontFeatureValues to the
list.

Review URL: https://codereview.chromium.org/444333005

git-svn-id: svn://svn.chromium.org/blink/trunk@179764 bbb929c8-8fbe-4397-9dbb-9b2b20218538