Fix blink_perf.dom select-single-add.html.

The test has never worked as expected.
The intention of the test is to measure the performance to add OPTIONs to a
SELECT element by
 - Adding 250 selected OPTIONs, and
 - Adding 250 unselected OPTIONs.

But it worked expectedly only at the first run. Actually, 499 unselected OPTIONs
and 1 selected OPTION were added after the first run because appendChild()
updates selected state.

This CL makes select-single-add.html slower intentionally.
This CL doesn't affect production.

BUG=625897
NOTRY=true

Review-Url: https://codereview.chromium.org/2119413002
Cr-Commit-Position: refs/heads/master@{#403835}
1 file changed