[roll third_party/pigweed] pw_async2: Implement initial async API

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/182727

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4070e04116fd0cb..f4abfe27a93e7c7
Roller-URL: https://ci.chromium.org/b/8761865559197458417
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4f8dc5ad779c916d1f901f160f8771e802fb4847
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/184855
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 4070e04..f4abfe2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4070e04116fd0cb8453a06aad35ebc6a44ddd8c2
+Subproject commit f4abfe27a93e7c7af1053184d310c8445f7b0a2d