DevTools: Check duplicate sources in dependency trees when compiling frontend

Drive-by: Use os.path.join() instead of manual string concatenation

R=vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/247203002

git-svn-id: svn://svn.chromium.org/blink/trunk@172130 bbb929c8-8fbe-4397-9dbb-9b2b20218538