Minimize performance regression caused by r157508

Minimize performance regression caused by r157508:
- Stop copying the value returned by Attribute::name()
- Cache the value returned by Element::attributeItem() as it hides
  an if branch in ElementData::attributeBase().

I get a 8.77% improvement locally on Bindings/id-setter.html 
performance test.

R=haraken
BUG=293663

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

git-svn-id: svn://svn.chromium.org/blink/trunk@161383 bbb929c8-8fbe-4397-9dbb-9b2b20218538