Don't transition a property that is currentColor when color changes

As per css3-color[1], currentColor is not resolved at computed value
time. This means that we shouldn't fire a transition when a property is
set to currentColor, even if the used value changes. Note we still
transition between currentColor and actual colours, which is likely to
be fine by css4-color, which adds a blending primitive.

[1] http://www.w3.org/Style/2011/REC-css3-color-20110607-errata.html

BUG=394632

Review URL: https://codereview.chromium.org/552683002

git-svn-id: svn://svn.chromium.org/blink/trunk@181546 bbb929c8-8fbe-4397-9dbb-9b2b20218538