Avoid recalculation of style attributes during box painting.

Some of these things, like background color, are non-trivial to compute (taking
into account :visited and so on); avoiding duplicate work yields about a 2% win
on recording time. (As measured on Linux desktop, but I expect it to be similar
elsewhere).

Review URL: https://codereview.chromium.org/410323002

git-svn-id: svn://svn.chromium.org/blink/trunk@178870 bbb929c8-8fbe-4397-9dbb-9b2b20218538