Convert string16 to base::string16 in content.

content/renderer/media/webcontentdecryptionmodule_impl.cc

Index: content/renderer/media/webcontentdecryptionmodule_impl.cc
diff --git a/content/renderer/media/webcontentdecryptionmodule_impl.cc b/content/renderer/media/webcontentdecryptionmodule_impl.cc
index 2c363ce4f78f8b6f8f752e8b19cdf6383b2a5c19..d3201247bcdcb33c4f525d857da22d2f8af24695 100644
--- a/content/renderer/media/webcontentdecryptionmodule_impl.cc
+++ b/content/renderer/media/webcontentdecryptionmodule_impl.cc
@@ -149,7 +149,7 @@ WebContentDecryptionModuleSessionImpl* ReferenceIdAdapter::GetSession(
 //------------------------------------------------------------------------------
 
 WebContentDecryptionModuleImpl*
-WebContentDecryptionModuleImpl::Create(const string16& key_system) {
+WebContentDecryptionModuleImpl::Create(const base::string16& key_system) {
   // TODO(ddorwin): Guard against this in supported types check and remove this.
   // Chromium only supports ASCII key systems.
   if (!IsStringASCII(key_system)) {