Reland "Add Abseil to third_party."

This is a reland of 816226e822635011c4e9dae6fb1d5ed8113bfbfd

Original change's description:
> 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>

Bug: skia:10165, b/154848688
Change-Id: I618bb4411445fe5b45a91741934ca888a09adf05
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285537
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
5 files changed