Revert 161383 "Minimize performance regression caused by r157508"

Caused crashes running layout tests on Win and Linux debug bots; see
Issue 293663 for details.

> 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

TBR=ch.dumez@samsung.com

Review URL: https://codereview.chromium.org/60273005

git-svn-id: svn://svn.chromium.org/blink/trunk@161401 bbb929c8-8fbe-4397-9dbb-9b2b20218538