media: Add UMA to report CDM interface version being used

media/cdm/ppapi/ppapi_cdm_adapter.cc

Index: media/cdm/ppapi/ppapi_cdm_adapter.cc
diff --git a/media/cdm/ppapi/ppapi_cdm_adapter.cc b/media/cdm/ppapi/ppapi_cdm_adapter.cc
index a28d7037db85777666dbd642ffa4ceedd4415e68..fe76959c5bd4ed82bfa219684904c557cd586c7a 100644
--- a/media/cdm/ppapi/ppapi_cdm_adapter.cc
+++ b/media/cdm/ppapi/ppapi_cdm_adapter.cc
@@ -355,6 +355,19 @@ CdmWrapper* PpapiCdmAdapter::CreateCdmInstance(const std::string& key_system) {
   DLOG_TO_CONSOLE(message);
   CDM_DLOG() << message;
 
+  if (cdm) {
+    pp::UMAPrivate uma_interface(this);
+
+    // The interface version is relatively small. So using normal histogram
+    // instead of a sparse histogram is okay. The following DCHECK asserts this.
+    PP_DCHECK(cdm->GetInterfaceVersion() <= 30);
+    uma_interface.HistogramEnumeration(
+        "Media.EME.CdmInterfaceVersion", cdm->GetInterfaceVersion(),
+        // Sample value should always be less than the boundary. Hence use "+1".
+        // See the comment on HistogramEnumeration in ppb_uma_private.idl.
+        cdm::ContentDecryptionModule::kVersion + 1);
+  }
+
   return cdm;
 }