switching checkout to git submodules

[Mike Frysinger]

i have a change pending to add git submodules support to breakpad.  hopefully existing gclient users should have a seamless transition, but i'll be honest, i really have no idea :).

this matches what we've done in other Chromium projects, and hopefully will make breakpad more approachable for non-Chromium folks as they can use standard git flows to get the project.

  git clone --recurse-submodules https://chromium.googlesource.com/breakpad/breakpad

i'll hold off on submitting for a bit to see if anyone has concerns.

  https://crrev.com/c/7247694

the general gclient/Chromium docs on submodules can be found here:

https://chromium.googlesource.com/chromium/src/+/HEAD/docs/git_submodules.md

-mike