Revert "Delete NO_MAIN_THREAD_WRAPPING preprocessor define."

This reverts commit 0f78c6b28dbc0c9caa555ce89ce91b0f08c510ea.

Reason for revert: Breaks downstream tests.

Original change's description:
> Delete NO_MAIN_THREAD_WRAPPING preprocessor define.
> 
> Since many tests rely on rtc::Thread::Current(), add an
> explicit rtc::AutoThread in the main() function used by tests.
> 
> Bug: webrtc:9714
> Change-Id: Id82121967c9621fe1c2945846009c48139fa57da
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/39680
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28000}

TBR=kwiberg@webrtc.org,nisse@webrtc.org,kthelgason@webrtc.org

Change-Id: Iff939bb0d5ad0ea01b953321993733bb56c9070b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9714
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/137512
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28001}

sdk/objc/unittests/main.mm

@@ -10,14 +10,11 @@
 
 #import <Foundation/Foundation.h>
 #import <UIKit/UIKit.h>
-#include "rtc_base/thread.h"
 #include "test/ios/coverage_util_ios.h"
 
 int main(int argc, char* argv[]) {
   rtc::test::ConfigureCoverageReportPath();
 
-  rtc::AutoThread main_thread;
-
   @autoreleasepool {
     return UIApplicationMain(argc, argv, nil, nil);
   }