Add tracing to VCMGenericEncoder::Release.

This call can take time, especially on mobile HW encoders. Make sure it
shows up in tracing.

BUG=webrtc:5167
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1606413002 .

Cr-Commit-Position: refs/heads/master@{#11374}

webrtc/modules/video_coding/generic_encoder.cc

@@ -109,6 +109,7 @@ VCMGenericEncoder::VCMGenericEncoder(
 VCMGenericEncoder::~VCMGenericEncoder() {}
 
 int32_t VCMGenericEncoder::Release() {
+  TRACE_EVENT0("webrtc", "VCMGenericEncoder::Release");
   return encoder_->Release();
 }