Cleanup: Move loading complete function into callback, rather than global.
https://bugs.webkit.org/show_bug.cgi?id=112167

Reviewed by Dirk Pranke.

Stop calling global functions, pass in proper handler to Loader
constructor. Note that the callback is optional, as tests, and
some potential dashboards, do not need one.

* TestResultServer/static-dashboards/dashboard_base.js:
* TestResultServer/static-dashboards/loader.js:
(.):
* TestResultServer/static-dashboards/loader_unittests.js:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145586 268f45cc-cd09-0410-ab3c-d52691b4dbfc