Add StyleBuilderConverter::convertColor to reduce copied code

This patch adds a color converter for -webkit-tap-highlight-color to use
and to reduce copied conversions for other color properties.

BUG=374530

Review URL: https://codereview.chromium.org/437453003

git-svn-id: svn://svn.chromium.org/blink/trunk@179283 bbb929c8-8fbe-4397-9dbb-9b2b20218538