Fix the compile error for win64: widevine_key_systems.cc.

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 d42ec7d680c7cd268e6a37cfd0655a85f1cc176c..c913fe090e95593901e663b248e8a516a44e0b50 100644
--- a/components/cdm/renderer/widevine_key_systems.cc
+++ b/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,