Improve update from chromium script

*) Inlines rev.py into update_from_chromium.py, no need to have it
separate
*) Adds //.clang-format to files to clone
*) Adds argparse so "--help" works as expected
*) Filter gyp_environment out of landmines.py to avoid false triggering
on gyp_generator_flags
*) use cwd param to subprocess instead of calling os.chdir all over the place

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/691443003