https://bugs.webkit.org/show_bug.cgi?id=97510

Reviewed by Benjamin Poulain.

PerformanceTests: 

In order to control the number of iterations and processes to use from run-perf-tests, always use 20
iterations on all tests except Dromaeo, where even doing 5 iterations is prohibitively slow, by default.
Without this change, it'll become extremely hard for us to tweak the number of iterations and processes
to use from run-perf-tests.

* Animation/balls.html:
* DOM/DOMTable.html:
* DOM/resources/dom-perf.js:
(runBenchmarkSuite.PerfTestRunner.measureTime):
* Dromaeo/resources/dromaeorunner.js:
* Layout/floats_100_100.html:
* Layout/floats_100_100_nested.html:
* Layout/floats_20_100.html:
* Layout/floats_20_100_nested.html:
* Layout/floats_2_100.html:
* Layout/floats_2_100_nested.html:
* Layout/floats_50_100.html:
* Layout/floats_50_100_nested.html:
* Layout/subtree-detaching.html:
* Parser/html5-full-render.html:
* SVG/SvgHitTesting.html:
* resou...