DevTools: [JsDocValidator] Make sure the jar is always updated when source files change

Added a PRESUBMIT.py check and rewrote the artifact builder in Python to reuse
file-hashing code.

R=aandrey, eustas, vsevik

Review URL: https://codereview.chromium.org/114043004

git-svn-id: svn://svn.chromium.org/blink/trunk@164299 bbb929c8-8fbe-4397-9dbb-9b2b20218538