DevTools: Check duplicate sources in dependency trees when compiling frontend
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@172130 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs