update paths to ref file support files when its location changes relative to the test file

With the recent W3C test repo refactoring, some assumptions re: support files from relative paths were falsified. This patch update paths to support files in ref files that change location relative to the test file upon import.

This patch is an updated version of this one, which was never commited: https://codereview.chromium.org/540703002/

This updated patch is in parity with its counterpart in Webkit that went through further review and landed. See: https://bugs.webkit.org/show_bug.cgi?id=135929

BUG=404845

Review URL: https://codereview.chromium.org/634373002

git-svn-id: svn://svn.chromium.org/blink/trunk@183497 bbb929c8-8fbe-4397-9dbb-9b2b20218538