[Perf tests] Reduce the number of loops in dom-attributes.html

[Perf tests] Reduce the number of loops in dom-attributes.html
https://bugs.webkit.org/show_bug.cgi?id=80077

Reviewed by Ryosuke Niwa.

dom-attributes.html takes 100~ seconds in the perf bots. This patch reduces
the time by reducing the number of loops.

This is just a temporary fix. After more detailed investigation, I will reduce
the number of tests in dom-attributes.html, and revert back the number of loops
to guarantee the reliability of the perf test results.

* Bindings/dom-attributes.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@110009 268f45cc-cd09-0410-ab3c-d52691b4dbfc