1. Add third_party/abseil-cpp to DEPS so it can be used in ANGLE.
2. Add abseil directory to ANGLE overrides.
3. Add third_party/abseil-cpp to .gitignore.
4. Add third_party/abseil-cpp to roll_chromium_deps.py so it's
auto-rolled periodically.

Bug: angleproject:4873
Change-Id: Ic158bba844e88ec90ac56a2f8c817168c4bea17f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2321734
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>