Add Abseil dependency for Dawn

Dawn is adding an dependency on Abseil, and a previous attempt to do so
broke the Dawn/Skia roller. Adding the library to DEPS here to support
that, though it will only be called from Dawn code (like Tint).

See: https://dawn-review.googlesource.com/c/dawn/+/64600

Bug: dawn:563
Change-Id: I002a086b3c8406da2a59d04321e9189f09e9e29a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/451638
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed