[roll third_party/pigweed] pw_multibuf: Add Chunk::Merge

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/177636

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 3964e532b2a7a11..5363b1f02a05690
Roller-URL: https://ci.chromium.org/b/8766237371198391809
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I80fccfa4f15ef85c964705a9db9fa3d9cbb2e0d5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/kudzu/+/177793
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 3964e53..5363b1f 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 3964e532b2a7a11667e8f90ca8e841fb56f3036c
+Subproject commit 5363b1f02a05690e6e1d5e7f3d53935ad0a2f110