[roll third_party/pigweed] pw_stream: Construct MemoryWriter w/prepopulated buffer

No-Docs-Update-Reason: Adding helper constructor, usage in header.
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48720

third_party/pigweed Rolled-Commits: 78d0730d24cfb89..5b5d9dd0f94d0b2
Roller-URL: https://ci.chromium.org/b/8844673120098681120
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8844673120098681120
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9b01d0d114853e4384222716becdc52f8449ff9a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/48687
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 78d0730..5b5d9dd 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 78d0730d24cfb893bc8cd0cfad9a5521fa55feae
+Subproject commit 5b5d9dd0f94d0b2463738555578fabee0448586a