Add decrypt-only external clear key browser tests.

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 97cc9e87a4e0b9e1900edc51282ea2a13ff86cd3..b9eceb2f4db139b1b47673b2a2e8c90df0b1a55d 100644
--- a/media/cdm/key_system_names.cc
+++ b/media/cdm/key_system_names.cc
@@ -4,10 +4,24 @@
 
 #include "media/cdm/key_system_names.h"
 
+#include <string>
+
 namespace media {
 
 const char kPrefixedClearKey[] = "webkit-org.w3.clearkey";
 const char kUnprefixedClearKey[] = "org.w3.clearkey";
 const char kExternalClearKey[] = "org.chromium.externalclearkey";
 
+static bool IsParentKeySystemOf(const std::string& parent_key_system,
+                                const std::string& key_system) {
+  std::string prefix = parent_key_system + '.';
+  return key_system.substr(0, prefix.size()) == prefix;
+}
+
+
+bool IsExternalClearKey(const std::string& key_system) {
+  return key_system == kExternalClearKey ||
+         IsParentKeySystemOf(kExternalClearKey, key_system);
+}
+
 }  // namespace media