[Performance test] Add a micro benchmark for div.firstChild getter

[Performance test] Add a micro benchmark for div.firstChild getter
https://bugs.webkit.org/show_bug.cgi?id=86582

Reviewed by Ryosuke Niwa.

We are going to remove Bindings/dom-attributes.html and instead add more
reasonable micro benchmarks by classifying DOM binding call paths.

In this bug, we add a micro benchmark for div.firstChild getter.
This benchmark covers 'firstChild', 'lastChild', 'nextSibling' and
'previousSibling' in Dromaeo/dom-traverse.html, and other DOM attributes
that return a Node object.

Multiple test results in my Linux desktop:

RESULT Bindings: first-child= 798.157160346 runs/s
median= 798.004987531 runs/s, stdev= 1.52006063407 runs/s, min= 796.019900498 runs/s, max= 801.001251564 runs/s
RESULT Bindings: first-child= 797.603608554 runs/s
median= 797.872340426 runs/s, stdev= 2.2522621261 runs/s, min= 791.556728232 runs/s, max= 801.001251564 runs/s
RESULT Bindings: first-child= 798.656295468 runs/s
median= 798.004987531 runs/s, stdev= 1.79367478063 runs/s, min= 797.01120797 runs/s, max= 803.011292346 runs/s
RESULT Bindings: first-child= 797.812784267 runs/s
median= 798.004987531 runs/s, stdev= 2.31766523191 runs/s, min= 791.100123609 runs/s, max= 802.005012531 runs/s
RESULT Bindings: first-child= 797.963311597 runs/s
median= 798.004987531 runs/s, stdev= 2.53014907337 runs/s, min= 789.14919852 runs/s, max= 801.001251564 runs/s

* Bindings/first-child.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117430 268f45cc-cd09-0410-ab3c-d52691b4dbfc