Simplify HTMLStyleElement.

The parser will never insert more than one child into a <style>, so we don't
need explicit logic to batch up parsing. Once I removed that it exposed that
all the line number and parser created logic is from error reporting in the
parser that's been dead in Blink for over a year.

By doing this simplification I was able to remove the finishParsingChildren()
callback entirely.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/788113002