Fix inconsistent setting of the rtc_include_unittests flag.

build_overrides/webrtc.gni

Index: build_overrides/webrtc.gni
diff --git a/build_overrides/webrtc.gni b/build_overrides/webrtc.gni
index 7891f3fe9857f78bb98a5ea8a7e1f1d8c14fd7dc..90e437423c69c3c3576b0cf380890cc51549e284 100644
--- a/build_overrides/webrtc.gni
+++ b/build_overrides/webrtc.gni
@@ -23,5 +23,8 @@ 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
+# There's no point processing WebRTC standalone tests in a Chromium build,
+# so give Chromium the option of overriding it here.
+declare_args() {
+  rtc_include_tests = true
+}