Prepare for deletion of the NO_MAIN_THREAD_WRAPPING preprocessor define

This is a partial reland of
https://webrtc-review.googlesource.com/c/src/+/39680,
including only the (hopefully) non-problematic parts of it, but
postponing actual deletion of automatic thread wrapping.

Bug: webrtc:9714
Change-Id: I9b79dd073f0e945cbb62f3b54cff05eaaea9b06c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/141664
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28265}

sdk/objc/unittests/main.mm

@@ -10,11 +10,14 @@
 
 #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);
   }