Skip unit test if GYP_DEFINES="rtc_use_h264=1" is not set.

webrtc/media/BUILD.gn

Index: webrtc/media/BUILD.gn
diff --git a/webrtc/media/BUILD.gn b/webrtc/media/BUILD.gn
index 9689cd2f1233a394bbdec38e32382220f5bb9f89..8f5a8c7405b0cdd8b8fa8daa47018bfe5ab0a4c7 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -288,6 +288,7 @@ if (rtc_include_tests) {
   test("rtc_media_unittests") {
     testonly = true
 
+    defines = []
     deps = []
     sources = [
       "base/codec_unittest.cc",
@@ -319,6 +320,9 @@ if (rtc_include_tests) {
     ]
     public_configs = [ "..:common_inherited_config" ]
 
+    if (rtc_use_h264) {
+      defines += [ "WEBRTC_USE_H264" ]
+    }
     if (is_win) {
       cflags = [
         "/wd4245",  # conversion from int to size_t, signed/unsigned mismatch.