Implement PayloadUnion as variant instead of pair of optionals

Bug: None
Change-Id: I2e54f5a0561804bc59c4d4c8e35ccdaa9536b8e4
Reviewed-on: https://webrtc-review.googlesource.com/85366
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23745}
diff --git a/DEPS b/DEPS
index 18eeb92..fe28211 100644
--- a/DEPS
+++ b/DEPS
@@ -745,4 +745,5 @@
 
   # Abseil whitelist.
   "+absl/types/optional.h",
+  "+absl/types/variant.h",
 ]