Fix the compile error for win64: widevine_key_systems.cc.

BUG=372069

Review URL: https://codereview.chromium.org/280063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269677 0039d316-1c4b-4281-b951-d872f2087c98

components/cdm/renderer/widevine_key_systems.cc

@@ -23,9 +23,9 @@ namespace cdm {
 
 // Return |name|'s parent key system.
 static std::string GetDirectParentName(std::string name) {
-  int last_period = name.find_last_of('.');
-  DCHECK_GT(last_period, 0);
-  return name.substr(0, last_period);
+  size_t last_period = name.find_last_of('.');
+  DCHECK_GT(last_period, 0u);
+  return name.substr(0u, last_period);
 }
 
 void AddWidevineWithCodecs(WidevineCdmType widevine_cdm_type,