Encrypted Media: Add UMA for system code.

Choose to put the UMA reporting code in content_decryptor_delegate.cc because:
- It's hard to do UMA in CdmAdapter due to pp::UMAPrivate limitations.
- I want to report system code for OnPromiseRejected() and OnSessionError() in
  one file. So I can't do it in CdmPromise.

BUG=412987
TEST=Tested by hacking Chromium to provide a wrong license. Then check about://histograms and I see the histogram generated.

Review URL: https://codereview.chromium.org/561643002

Cr-Commit-Position: refs/heads/master@{#294478}

content/renderer/pepper/content_decryptor_delegate.cc

@@ -7,7 +7,9 @@
 #include "base/callback_helpers.h"
 #include "base/debug/trace_event.h"
 #include "base/message_loop/message_loop_proxy.h"
+#include "base/metrics/sparse_histogram.h"
 #include "base/numerics/safe_conversions.h"
+#include "content/renderer/media/crypto/key_systems.h"
 #include "content/renderer/pepper/ppb_buffer_impl.h"
 #include "media/base/audio_buffer.h"
 #include "media/base/audio_decoder_config.h"
@@ -94,7 +96,7 @@ bool CopyStringToArray(const std::string& str, uint8 (&array)[array_size]) {
 //
 // Returns true if |block_info| is successfully filled. Returns false
 // otherwise.
-static bool MakeEncryptedBlockInfo(
+bool MakeEncryptedBlockInfo(
     const scoped_refptr<media::DecoderBuffer>& encrypted_buffer,
     uint32_t request_id,
     PP_EncryptedBlockInfo* block_info) {
@@ -288,6 +290,16 @@ MediaKeys::Exception PpExceptionTypeToMediaException(
   }
 }
 
+// TODO(xhwang): Unify EME UMA reporting code when prefixed EME is deprecated.
+// See http://crbug.com/412987 for details.
+void ReportSystemCodeUMA(const std::string& key_system, uint32 system_code) {
+  // Sparse histogram macro does not cache the histogram, so it's safe to use
+  // macro with non-static histogram name here.
+  UMA_HISTOGRAM_SPARSE_SLOWLY(
+      "Media.EME." + KeySystemNameForUMA(key_system) + ".SystemCode",
+      system_code);
+}
+
 }  // namespace
 
 ContentDecryptorDelegate::ContentDecryptorDelegate(
@@ -767,6 +779,8 @@ void ContentDecryptorDelegate::OnPromiseRejected(
     PP_CdmExceptionCode exception_code,
     uint32 system_code,
     PP_Var error_description) {
+  ReportSystemCodeUMA(key_system_, system_code);
+
   StringVar* error_description_string = StringVar::FromPPVar(error_description);
   DCHECK(error_description_string);
 
@@ -846,6 +860,8 @@ void ContentDecryptorDelegate::OnSessionError(
     PP_CdmExceptionCode exception_code,
     uint32 system_code,
     PP_Var error_description) {
+  ReportSystemCodeUMA(key_system_, system_code);
+
   if (session_error_cb_.is_null())
     return;
 

tools/metrics/histograms/histograms.xml

@@ -11305,6 +11305,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>KeyError event count using an unknown key system.</summary>
 </histogram>
 
+<histogram name="Media.EME.Unknown.SystemCode" units="system code">
+  <owner>xhwang@chromium.org</owner>
+  <summary>System code count using an unknown key system.</summary>
+</histogram>
+
 <histogram name="Media.EME.Widevine.addKey" enum="MediaKeyException">
   <owner>xhwang@chromium.org</owner>
   <summary>addKey result using the Widevine key system.</summary>
@@ -11343,6 +11348,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>KeyError event count using the Widevine key system.</summary>
 </histogram>
 
+<histogram name="Media.EME.Widevine.SystemCode" units="system code">
+  <owner>xhwang@chromium.org</owner>
+  <summary>System code count using the Widevine key system.</summary>
+</histogram>
+
 <histogram name="Media.FallbackHardwareAudioBitsPerChannel">
   <owner>dalecurtis@chromium.org</owner>
   <summary>