Flakiness dashboard shows the wrong master's data when builder names are duplicated.

builders.master only queries using the builder name. So, only populate
it with the list of builders for the currently loaded group instead of
trying to have it be a list of all possible builders.

BUG=372612
NOTRY=true

Review URL: https://codereview.chromium.org/283283002

git-svn-id: svn://svn.chromium.org/blink/trunk@174062 bbb929c8-8fbe-4397-9dbb-9b2b20218538