const, reference and simplify VisitedLinkState

Bring VisitedLinkState into the future.

I also removed a !frame() check from VisitedLinkState that didn't make
sense. The comment claimed it was for avoiding determining the link state
for elements in XHR documents but we only ever call determineLinkState from
inside style recalc and entering style recalc without a frame() would be
catastrophic (and never happens). Instead I added an ASSERT that the document
isActive() here which is true for any document entering recalcStyle.

Review URL: https://codereview.chromium.org/27156002

git-svn-id: svn://svn.chromium.org/blink/trunk@159609 bbb929c8-8fbe-4397-9dbb-9b2b20218538