Add Abseil to third_party.

At present, this is a proof-of-concept which only supports a small
subset of absl modules:
- Base
- Hash
- Numeric
- String

This is only used by one unit test, which builds a string and then
hashes it.

Bug: skia:10165
Bug: b/154848688

Change-Id: I016250bf700b522c7a6bc78cf1844abff2260c35
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/284805
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
5 files changed