Only show the platform-appropriate builders for non-layout test failures in garden...

Only show the platform-appropriate builders for non-layout test failures in garden-o-matic
https://bugs.webkit.org/show_bug.cgi?id=90025

Reviewed by Simon Fraser.

Move the chromium-specific filtering code into config.js and replace it with a method on each platform
config. Also, let the webkit test step name be webkit_tests (build.chromium.org) or layout-test (build.webkit.org).

* BuildSlaveSupport/build.webkit.org-config/public_html/TestFailures/scripts/builders.js:
* BuildSlaveSupport/build.webkit.org-config/public_html/TestFailures/scripts/builders_unittests.js:
* BuildSlaveSupport/build.webkit.org-config/public_html/TestFailures/scripts/config.js:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121301 268f45cc-cd09-0410-ab3c-d52691b4dbfc