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  a=blocking-basecamp

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.
Fix bustage from bug 822721.