Make checkdeps.py check third_party.

For this first cl, it's only checking checked in code. I've had to add DEPS files to make it pass, since some of this code legitimately uses base etc...

BUG=469692
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/1045863002

Cr-Original-Commit-Position: refs/heads/master@{#323302}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5b6373fbaed471c766ec691ad2c816abbc482d29