Optimize parseHTMLInteger()

Optimize parseHTMLInteger()
https://bugs.webkit.org/show_bug.cgi?id=154772

Reviewed by Ryosuke Niwa.

Optimize parseHTMLInteger() a bit now that it is used a lot more and
that it has decent API test coverage. In particular, we now:
- Avoid using a StringBuilder for the digits.
- Get rid of a is8Bit() branch.
- Only traverse the input string once.

* html/parser/HTMLParserIdioms.cpp:
(WebCore::parseHTMLIntegerInternal):
(WebCore::parseHTMLInteger):
(WebCore::parseHTMLNonNegativeInteger): Deleted.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@197255 268f45cc-cd09-0410-ab3c-d52691b4dbfc