Make rtc_include_tests true by default in gn.

build_overrides/webrtc.gni

Index: build_overrides/webrtc.gni
diff --git a/build_overrides/webrtc.gni b/build_overrides/webrtc.gni
index 7891f3fe9857f78bb98a5ea8a7e1f1d8c14fd7dc..0192515d90dfadcce6047499ced8a4d6753edd5b 100644
--- a/build_overrides/webrtc.gni
+++ b/build_overrides/webrtc.gni
@@ -23,5 +23,7 @@ rtc_include_pulse_audio = true
 # standalone WebRTC.
 rtc_include_internal_audio_device = true
 
-# There's no point processing WebRTC standalone tests in a Chromium build.
-rtc_include_tests = true
+declare_args() {
+  # Include tests in standalone checkout.
+  rtc_include_tests = true
+}