Allow the run_code_generation.py script to be run from any directory. BUG=angleproject:2695 Change-Id: I9a083bf67b17ca0dc1ee213b75caa4ecece62f00 Reviewed-on: https://chromium-review.googlesource.com/1127354 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>