Rename ElementData's attribute related methods for clarity

Rename ElementData's attribute related methods for clarity:
* length() -> attributeCount()
* isEmpty() -> hasAttributes()
* getAttributeItem(name) -> findAttributeByName(name)
* attributeItem(index) -> attributeAt(index)
* getAttributeItemIndex(name) -> findAttributeIndexByName(name)
* addAttribute(name, value) -> appendAttribute(name, value)
* removeAttribute(index) -> removeAttributeAt(index)

This is inpired by the following WebKit revision by benjamin@webkit.org:
http://trac.webkit.org/changeset/162394

R=esprehn@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/313973003

git-svn-id: svn://svn.chromium.org/blink/trunk@175611 bbb929c8-8fbe-4397-9dbb-9b2b20218538