Handle raw <h#> when calculating $rawtoc

When the parser is constructing $rawtoc, it needs the sectionIndex
number to be able to calculate the byteoffset. This number is only
available for wikitext headings ("== foo =="), HTML headings
("<h2>foo</h2>") do not have it and the lack makes byteoffset be wrong
for all subsequent headings in the page.

To fix this, we just omit output of byteoffset in this situation.

Bug: 25203
Change-Id: I39e5faa4ac22d915f06125aac36ced11607b94a3
Merge "Handle raw <h#> when calculating $rawtoc"