media: Only unregister a CDM when it has been registered.

A CDM is registered after it's succussfully created (so the CDM ID is valid).
Check the CDM ID to decide whether we need to unregister it.

Review URL: https://codereview.chromium.org/1603413004

Cr-Commit-Position: refs/heads/master@{#370786}

media/mojo/services/mojo_cdm_service.cc

@@ -91,9 +91,12 @@ MojoCdmService::MojoCdmService(
 }
 
 MojoCdmService::~MojoCdmService() {
+  if (cdm_id_ == CdmContext::kInvalidCdmId)
+    return;
+
   g_cdm_manager.Get().UnregisterCdm(cdm_id_);
 
-  if (cdm_id_ != CdmContext::kInvalidCdmId && context_)
+  if (context_)
     context_->UnregisterCdm(cdm_id_);
 }