Inline hot ElementData functions.

This is a merge of http://trac.webkit.org/changeset/156227 by
Andreas Kling <akling@apple.com>.

This patch inlines the following functions:
ElementData::deref
UniqueElementData::addAttribute
UniqueElementData::removeAttribute
UniqueElementData::attributeItem

In inlining these functions, two ASSERT_WITH_SECURITY_IMPLs have
been removed in attributeAt and removeAttribute. These functions
are accessing/removing items on Vector, we already have a
RELEASE_ASSERT for them in Vector.h.

This results in a 1.1% performance increase on
BlinkPerfMeasurement:Parser_html5-full-render

Review URL: https://codereview.chromium.org/58973002

git-svn-id: svn://svn.chromium.org/blink/trunk@161313 bbb929c8-8fbe-4397-9dbb-9b2b20218538