Add PerformanceTest for document startup via iframe append/remove

Add PerformanceTest for document startup via iframe append/remove
https://bugs.webkit.org/show_bug.cgi?id=111389

Reviewed by Kentaro Hara.

We'd like pages with lots of iframes to be fast.
This test shows that at least for V8/Chromium we spend
all our time booting v8.

On my 32-core linux machine I see about 70 runs/sec in
Release and 5 runs/sec in Debug.

* DOM/iframe-append-remove.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@144704 268f45cc-cd09-0410-ab3c-d52691b4dbfc