Outline Element::shouldIgnoreAttributeCase()

This allows the Document.h include to be removed from Element.h, which
reduces the amount of recompilation when touching Document.h by about
12%, from 1382 steps to 1214 steps locally.

The only place outside of Element.cpp that uses this function is
NamedNodeMap::removeNamedItem, which is very rarely used:
http://www.chromestatus.com/metrics/feature/timeline/popularity/308

In other words, the performance difference should not matter.

Review URL: https://codereview.chromium.org/334413005

git-svn-id: svn://svn.chromium.org/blink/trunk@176358 bbb929c8-8fbe-4397-9dbb-9b2b20218538