This makes changes nsDocument::mLinkMap in a number of ways:
1) renamed to mStyledLinks to better reflect its new nature.
2) change it to an nsTHashtable of Link*. It no longer has a strong reference
3) add some assertions to make sure we call ForgetLink and AddStyleRelevantLink
in pairs.
This also makes mozilla::dom::Link::ResetLinkState take a boolean indicating if
we should notify or not.
r=bz