[roll third_party/pigweed] pw_allocator: Return Range from SplitFreeListAllocator

This CL adds a method to get a Range of the blocks being tracked by a
SplitFreeListAllocator.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/177807

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 38b84d7e926aec5..feab5389df0005a
Roller-URL: https://ci.chromium.org/b/8766112298281961857
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I7a4a265326b6eedfa61bac579d95c16988dedb15
Reviewed-on: https://pigweed-review.googlesource.com/c/gonk/+/178031
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 38b84d7..feab538 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 38b84d7e926aec559bd0ba686de87d32f72b0581
+Subproject commit feab5389df0005ae9febc2f83f18f38e7df8bda0