Migrate WebRTC test infra to ABSL_FLAG.

This is the last CL required to migrate WebRTC to ABSL_FLAG, rtc::Flag
will be removed soon after this one lands.

Bug: webrtc:10616
Change-Id: I2807cec39e28a2737d2c49e2dc23f2a6f98d08f0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145727
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28606}

test/test_main_lib.h

@@ -11,6 +11,7 @@
 #define TEST_TEST_MAIN_LIB_H_
 
 #include <memory>
+#include <string>
 
 namespace webrtc {
 
@@ -31,6 +32,8 @@ class TestMain {
 
  protected:
   TestMain() = default;
+
+  std::string field_trials_;
 };
 
 }  // namespace webrtc