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

Unit test would fail in default configuration (e.g. rtc_use_h264=0), cause it tests instantiating H264 specifics.

BUG=webrtc:6194, webrtc:6198

Review-Url: https://codereview.webrtc.org/2228733004
Cr-Commit-Position: refs/heads/master@{#13929}
diff --git a/webrtc/media/BUILD.gn b/webrtc/media/BUILD.gn
index 7793dfa..e2d9217 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -290,6 +290,7 @@
   test("rtc_media_unittests") {
     testonly = true
 
+    defines = []
     deps = []
     sources = [
       "base/codec_unittest.cc",
@@ -322,6 +323,9 @@
     ]
     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.