Don't assume that builds finish in the order they were started

Don't assume that builds finish in the order they were started

Fixes <http://webkit.org/b/61464> TestFailures page shows failures from a not-most-recent
build when one slave is taking a long time to finish a build

Reviewed by Antti Koivisto.

* BuildSlaveSupport/build.webkit.org-config/public_html/TestFailures/Builder.js:
(Builder.prototype.getMostRecentCompletedBuildNumber): Don't assume that all builds that are
newer than the oldest in-progress build are also themselves in progress. Just check whether
each build is actually in progress (as signified by its presence in the currentBuilds set).

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87380 268f45cc-cd09-0410-ab3c-d52691b4dbfc