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}
diff --git a/BUILD.gn b/BUILD.gn
index 4a04def..95d02f7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -83,6 +83,13 @@
   }
 }
 
+# Abseil Flags by default doesn't register command line flags on mobile
+# platforms, WebRTC tests requires them (e.g. on simualtors) so this
+# config will be applied to testonly targets globally (see webrtc.gni).
+config("absl_flags_configs") {
+  defines = [ "ABSL_FLAGS_STRIP_NAMES=0" ]
+}
+
 config("library_impl_config") {
   # Build targets that contain WebRTC implementation need this macro to
   # be defined in order to correctly export symbols when is_component_build