Adding support for color interpolation hints to CSS gradients.

CSS images level 4 defines support for gradient color interpolation
hints (aka midpoints):
http://dev.w3.org/csswg/css-images-4/#color-interpolation-hint

Webkit: https://bugs.webkit.org/show_bug.cgi?id=137171
Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1074056

Manual test: http://codepen.io/adobe/full/fhnBJ

BUG=420527

Review URL: https://codereview.chromium.org/631753002

git-svn-id: svn://svn.chromium.org/blink/trunk@183460 bbb929c8-8fbe-4397-9dbb-9b2b20218538