Ban absl::StrSplit and absl::StrJoin

For binary size reasons; they emit too many instructions at each call
site.

I decided to not point out replacements for them, since rtc::join has
only one caller, and rtc::split isn't the only option for splitting
strings. And both of them have APIs that can cause the creation of
needless temporary strings.

Bug: none
Change-Id: I06161012836a90b09249c2813d2ddc15f167d580
Notry: true
Reviewed-on: https://webrtc-review.googlesource.com/c/124480
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26846}

abseil-in-webrtc.md

@@ -49,9 +49,9 @@ and avoid `absl::Span` until C++20 is finalized and the Abseil team
 has decided if they will change `absl::Span` to match.
 [Bug](https://bugs.webrtc.org/9214).
 
-### `absl::StrCat` and `absl::StrAppend`
+### `absl::StrCat`, `absl::StrAppend`, `absl::StrJoin`, `absl::StrSplit`
 
-*Use `rtc::SimpleStringBuilder` instead.*
+*Use `rtc::SimpleStringBuilder` to build strings.*
 
 These are optimized for speed, not binary size. Even `StrCat` calls
 with a modest number of arguments can easily add several hundred bytes