DevTools: Refactor timeline flamechart JS samples->frames converter

Drive-by fix: eliminate artifacts like JS frames hanging past their parents.

BUG=363976

Review URL: https://codereview.chromium.org/582003002

git-svn-id: svn://svn.chromium.org/blink/trunk@182313 bbb929c8-8fbe-4397-9dbb-9b2b20218538