Make currentColor behave the same internally as initial

While most color properties have an initial value of currentColor,
explicitly specifying currentColor behaves different internally to an
initial value. This patch makes currentColor no longer resolved in style
resolution for properties which are a single color and removes a flag
from RenderStyle indicating that currentColor was explicitly set.

Review URL: https://codereview.chromium.org/547273002

git-svn-id: svn://svn.chromium.org/blink/trunk@181538 bbb929c8-8fbe-4397-9dbb-9b2b20218538