Create unit tests for RenderTextPainter::textPaintingStyle.

There's some non-trivial logic here about resolving colors that
benefits from some simple unit tests.

Review URL: https://codereview.chromium.org/614623002

git-svn-id: svn://svn.chromium.org/blink/trunk@182840 bbb929c8-8fbe-4397-9dbb-9b2b20218538