Add new StyleColor class

Add new class for representing a color in the style/rendering systems.
Unlike the regular, lower level, Color class this one has the concept
of currentColor. The medium term plan is to also move the concept of
isValid from Color to StyleColor.

BUG=261854
R=dglazkov@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19749005

git-svn-id: svn://svn.chromium.org/blink/trunk@154615 bbb929c8-8fbe-4397-9dbb-9b2b20218538