Speed up ElementData::getAttributeItem, which is hot.
Speed up ElementData::getAttributeItem, which is hot.

The previous version had a couple of issues:
- Incurred new WTF::Vector::operator[] checks.
- Had a branch in _every_ loop iteration that checked whether the storage was Vector or array[] backed.

Both these issues are fixed, and CloneNodes.html seems reliably at +2%.
I also believe GetElement.html is maybe +1%.

R=inferno@chromium.org,haraken@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/13959007

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@148622 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs