Reland "Introduce RTC_NO_UNIQUE_ADDRESS."

This is a reland of f5e261aaf65cdf2eb903cdf40d651846be44f447

This CL disables RTC_NO_UNIQUE_ADDRESS on MSan builds since
there have been some issues.

Original change's description:
> Introduce RTC_NO_UNIQUE_ADDRESS.
>
> This macro introduces the possibility to suggest the compiler that a
> data member doesn't need an address different from other non static
> data members.
>
> The usage of a macro is to maintain portability since at the moment
> the attribute [[no_unique_address]] is only supported by clang
> with at least -std=c++11 but it should be supported by all the
> compilers starting from C++20.
>
> Bug: webrtc:11495
> Change-Id: I9f12b67b4422a2749649eaa6b004a67d5fd572d8
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173331
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#32246}

Bug: webrtc:11495, webrtc:12218
Change-Id: I4e6c7cc37d3daffad2407c9a2acfa897fa5b426a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/189968
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32668}
diff --git a/rtc_base/net_helpers.h b/rtc_base/net_helpers.h
index c6aa4be..172a222 100644
--- a/rtc_base/net_helpers.h
+++ b/rtc_base/net_helpers.h
@@ -23,6 +23,7 @@
 #include "rtc_base/ip_address.h"
 #include "rtc_base/socket_address.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/system/rtc_export.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "rtc_base/thread.h"
@@ -62,7 +63,7 @@
   bool recursion_check_ =
       false;  // Protects against SignalDone calling into Destroy.
   bool destroy_called_ = false;
-  webrtc::SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS webrtc::SequenceChecker sequence_checker_;
 };
 
 // rtc namespaced wrappers for inet_ntop and inet_pton so we can avoid