DevTools: scripts: remove manual path concatenation / splitting

Use python platform-dependent methods for this.

+ cleanup imports
+ fix execution bit

BUG=425468

Review URL: https://codereview.chromium.org/662393004

git-svn-id: svn://svn.chromium.org/blink/trunk@184158 bbb929c8-8fbe-4397-9dbb-9b2b20218538