Revert of Pass initDataType instead of contentType on OnNeedKey().

content/renderer/media/crypto/encrypted_media_player_support_impl.cc

Index: content/renderer/media/crypto/encrypted_media_player_support_impl.cc
diff --git a/content/renderer/media/crypto/encrypted_media_player_support_impl.cc b/content/renderer/media/crypto/encrypted_media_player_support_impl.cc
index e3f70b393d18d6e316057648e99da1f20219f456..42f86b6e461d9558a58515dce01ebf8cab9703e2 100644
--- a/content/renderer/media/crypto/encrypted_media_player_support_impl.cc
+++ b/content/renderer/media/crypto/encrypted_media_player_support_impl.cc
@@ -117,11 +117,11 @@
 // so we keep it as simple as possible without breaking major use cases.
 static std::string GuessInitDataType(const unsigned char* init_data,
                                      unsigned init_data_length) {
-  // Most WebM files use KeyId of 16 bytes. CENC init data is always >16 bytes.
+  // Most WebM files use KeyId of 16 bytes. MP4 init data are always >16 bytes.
   if (init_data_length == 16)
-    return "webm";
-
-  return "cenc";
+    return "video/webm";
+
+  return "video/mp4";
 }
 
 scoped_ptr<media::EncryptedMediaPlayerSupport>