Refactor CTFailures._processFailuresForRangeGroup.

Avoid duplicate array traversals, and clarify what the code is trying to do.

Review URL: https://codereview.chromium.org/538293002

git-svn-id: svn://svn.chromium.org/blink/trunk@181593 bbb929c8-8fbe-4397-9dbb-9b2b20218538