DYEBench should run 20 iterations in browser

DYEBench should run 20 iterations in browser
https://bugs.webkit.org/show_bug.cgi?id=132795

Reviewed by Gavin Barraclough.

Right now, run-perf-tests runs BYEBench 4 times with different instances of DRT/WTR to get
a more stable time and account for differences in the runtime environment, particularly,
ASLR (Address Space Layout Randomization).

While we can't account for the latter effect when the benchmark is ran inside a browser,
we can at least run 20 iterations to get a more stable (but perhaps biased by ALSR) number.

While the sample size of 20 is statistically unsound to compute the arithmetic mean from,
it's MUCH better than the current sample size of 5.

* DoYouEvenBench/Full.html:
(benchmarkClient.iterationCount): Set the default iteration count to 20.
(startTest): Use benchmarkClient.iterationCount as the iteration count.
* DoYouEvenBench/resources/benchmark-report.js:
(benchmarkClient.iterationCount): Continue to use the old iteration count of 5 when ran
inside run-perf-tests.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168732 268f45cc-cd09-0410-ab3c-d52691b4dbfc