Add log when rejecting key system request without media capabilities.

media/blink/key_system_config_selector.cc

Index: media/blink/key_system_config_selector.cc
diff --git a/media/blink/key_system_config_selector.cc b/media/blink/key_system_config_selector.cc
index 7f35733665dc0ca66199522aaf2d7ba146ac079a..5fcd5675bbc0a0218e8695e5cfb3c00ab8ae8cc2 100644
--- a/media/blink/key_system_config_selector.cc
+++ b/media/blink/key_system_config_selector.cc
@@ -652,6 +652,8 @@ KeySystemConfigSelector::GetSupportedConfiguration(
   //     configuration are both empty, return NotSupported.
   if (candidate.videoCapabilities.isEmpty() &&
       candidate.audioCapabilities.isEmpty()) {
+    DVLOG(2) << "Rejecting requested configuration because "
+             << "neither audioCapabilities nor videoCapabilities is specified";
     return CONFIGURATION_NOT_SUPPORTED;
   }