Cleanup: Pass std::string as const reference if possible

components/cdm/renderer/widevine_key_systems.cc

Index: components/cdm/renderer/widevine_key_systems.cc
diff --git a/components/cdm/renderer/widevine_key_systems.cc b/components/cdm/renderer/widevine_key_systems.cc
index 39980667fa955176aa7d10c238fa956940a0d755..8cc639910864e67be88076f012651d5a8691f12f 100644
--- a/components/cdm/renderer/widevine_key_systems.cc
+++ b/components/cdm/renderer/widevine_key_systems.cc
@@ -20,7 +20,7 @@ using media::SupportedCodecs;
 namespace cdm {
 
 // Return |name|'s parent key system.
-static std::string GetDirectParentName(std::string name) {
+static std::string GetDirectParentName(const std::string& name) {
   size_t last_period = name.find_last_of('.');
   DCHECK_GT(last_period, 0u);
   return name.substr(0u, last_period);