Perf dashboard spends 2s processing JSON data during the page loads

Perf dashboard spends 2s processing JSON data during the page loads
https://bugs.webkit.org/show_bug.cgi?id=135152

Reviewed by Andreas Kling.

In the Apple internal dashboard, we were spending as much as 2 seconds
converting raw JSON data into proper JS objects while loading the dashboard.

This caused the apparent unresponsiveness of the dashboard despite of the fact
charts themselves updated almost instantaneously.

* public/index.html:
* public/js/helper-classes.js:
(TestBuild): Compute the return values of formattedTime and formattedBuildTime
lazily as creating new Date objects and running string replace is expensive.
(TestBuild.formattedTime):
(TestBuild.formattedBuildTime):
(PerfTestRuns.setResults): Added. Pushing each result was the biggest bottle neck.
(PerfTestRuns.addResult): Deleted.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171361 268f45cc-cd09-0410-ab3c-d52691b4dbfc