Move the third_party/protobuf deps to a subdirectory

We already have some files checked in in third_party and this is not a
problem when using gitdeps.py as it'll just merge the protobuf repo
in this directory, but this doesn't work when using regular DEPS (it
complains that the directory isn't empty and remove the checked-in
files).

Bug: 759092
Change-Id: Id61fc7455b2e411e6c4ca39dcf932e61bbddf423
Reviewed-on: https://chromium-review.googlesource.com/809644
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
4 files changed