Avoid insertions (e.g., beacon) at midpoint of the document which ought to be at the end of it

[GoogleCodeExporter]

Don't improperly call SummariesDone for every flush window in cases
involving filters deleting slots (e.g. CSS combiner) and multiple
flush windows --- it's supposed to be called only at the very end.

This could potentially lead to multiple beacon insertions, and also
to really confused HTML parser, since we were trying to do
Insertion-at-body-end at a midpoint of document, which spammed loadtest
with "HtmlElement Parents of ..." warnings.

Original issue reported on code.google.com by hui...@google.com on 6 Jan 2014 at 4:21