Change RenderTreeAsText to not lie about inset/outset borders

Update RenderTreeAsText to return the actual border color for
inset/outset borders instead of lying about it.
This also allows us to remove the RenderStyle friend declaration
for RenderTreeAsText.

BUG=263058
R=dglazkov@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19981002

git-svn-id: svn://svn.chromium.org/blink/trunk@154767 bbb929c8-8fbe-4397-9dbb-9b2b20218538