Minimize performance regression caused by r157508

Minimize performance regression caused by r157508:
- Cache the value returned by Element::attributeItem() as it hides
  an if branch in ElementData::attributeBase().
- Minimize the cost of calling attrIfExists() which does a
  linear search by using iterators (thus avoiding the release
  assertion in Vector::at())

I get a 3.1% improvement locally on Bindings/id-setter.html 
performance test.

R=haraken
BUG=293663

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=161383

Review URL: https://codereview.chromium.org/60143002

git-svn-id: svn://svn.chromium.org/blink/trunk@161419 bbb929c8-8fbe-4397-9dbb-9b2b20218538