Added support for appending an escaped SegmentedString to another.

This was previously an assert which triggers in the situation where the HTMLEntityParser is parsing a potential named entity, but only has one or two characters of data available due to being at the end of a network packet boundary, so it 'unconsumes' them from the SegmentedString which has some special handling when only one or two characters needs unconsuming to avoid string creation. Next time the tokenizer is pumped, the SegmentedString is then appended to the HTMLSourceTracker right before the actual parsing happens (i.e. when we'd consume the previously unconsumed characters). SegmentedString::append mostly assumes the parameter string hasn't been consumed from and had an assert instead of handling this case, which hits.

R=eseidel
BUG=345726

Review URL: https://codereview.chromium.org/298803012

git-svn-id: svn://svn.chromium.org/blink/trunk@174725 bbb929c8-8fbe-4397-9dbb-9b2b20218538