Bug 822721:  Call CalcStyleDifference and process the style change list resulting from the miniflush we do to update throttled animations prior to seeing if we need to start transitions.  r=bzbarsky

The CalcStyleDifference call is absolutely necessary even if we didn't
need to process the change list, because it causes the new style
context to have cached structs that we might need for a later
comparison.  This is important because, as an optimization, we only
compare structs that have been retrieved.  This optimization requires
that when we replace a style context, we fetch all the structs on the
new style context that had been fetched on the old style context (which
is normally necessary anyway in order to do comparison so we can process
the changes appropriately).

However, actually processing the change list is also necessary to fix
the bug; it's the actual change from the miniflush that matters here.
Based on dholbert's debugging information, I think it's mostly likely
because we were failing to process the UpdateOverflow hint.
Bug 827579:  Remove post-resolve callback concept from style system.  r=bzbarsky
Bug 827704:  Fix 'visibility' animation so that it doesn't animate between values when one of them is not 'visibile'.  r=bzbarsky