roll: third_party/pigweed pw_multibuf: Remove MultiBuf's Chunk iterator wrappers

Instead, use the iterators in MultiBufChunks directly. This avoids
having two different iterator types in one class.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/230893
Original-Revision: e31487f84caccbbfc986d8c6327c278ff1548c7d

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 6b20a021cf616e..e31487f84caccb
Roller-URL: https://ci.chromium.org/b/8736203094325120961
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic539eabdbed3c1df1f5e4a417adf5bc8453df96a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/237235
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 6b20a02..e31487f 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 6b20a021cf616e20749c3eeb9cdbf8ebff12dcca
+Subproject commit e31487f84caccbbfc986d8c6327c278ff1548c7d