We already supported plain headings without the 'mw-headline'
wrappers, but now we need to parse the 'id' from a different
attribute.
Needed-By: If04d72f427ec3c3730e757cbb3ade8840c09f7d3
Bug: T357723
Change-Id: If85f89c40834618f23dc0ace2e599efb3b6d5ed4