EME: Remove the concept of a parent key system.

media/cdm/key_system_names.cc

Index: media/cdm/key_system_names.cc
diff --git a/media/cdm/key_system_names.cc b/media/cdm/key_system_names.cc
index f0f1c2e7e9faffd431c7b2597d9526e10837b96b..bcddaea2043ba9047bef0d3860eefd6075621ca4 100644
--- a/media/cdm/key_system_names.cc
+++ b/media/cdm/key_system_names.cc
@@ -13,15 +13,15 @@ bool IsClearKey(const std::string& key_system) {
   return key_system == kClearKey;
 }
 
-bool IsParentKeySystemOf(const std::string& parent_key_system,
-                         const std::string& key_system) {
-  std::string prefix = parent_key_system + '.';
+bool IsChildKeySystemOf(const std::string& key_system,
+                        const std::string& base) {
+  std::string prefix = base + '.';
   return key_system.substr(0, prefix.size()) == prefix;
 }
 
 bool IsExternalClearKey(const std::string& key_system) {
   return key_system == kExternalClearKey ||
-         IsParentKeySystemOf(kExternalClearKey, key_system);
+         IsChildKeySystemOf(key_system, kExternalClearKey);
 }
 
 }  // namespace media