Chrome on iOS downstream repository tracks Chromium via DEPS and wants
to use cpplint canned presubmit check but the cpplint.py errors due at
the include guard as it stops at the inner most git repository.

Add a new parameter --project_root to cpplint.py. If set, it overrides
the automatic detection of the project root that searches the root of
the version control repository.

The --root parameter cannot be used as it is used after the automatic
resolution is performed (and allow chopping the head of the relative
path while the need is to expend the path to include ios_internal/).

BUG=598090

Review-Url: https://codereview.chromium.org/2036773002