Make AccessControl GSN compatible (#2135)

contracts/access/AccessControl.sol

@@ -164,7 +164,7 @@ abstract contract AccessControl is Context {
      */
     function _grantRole(bytes32 role, address account) internal virtual {
         if (_roles[role].members.add(account)) {
-            emit RoleGranted(role, account, msg.sender);
+            emit RoleGranted(role, account, _msgSender());
         }
     }
 
@@ -175,7 +175,7 @@ abstract contract AccessControl is Context {
      */
     function _revokeRole(bytes32 role, address account) internal virtual {
         if (_roles[role].members.remove(account)) {
-            emit RoleRevoked(role, account, msg.sender);
+            emit RoleRevoked(role, account, _msgSender());
         }
     }
 

contracts/mocks/AccessControlMock.sol

@@ -4,7 +4,7 @@ import "../access/AccessControl.sol";
 
 contract AccessControlMock is AccessControl {
     constructor() public {
-        _grantRole(DEFAULT_ADMIN_ROLE, msg.sender);
+        _grantRole(DEFAULT_ADMIN_ROLE, _msgSender());
     }
 
     function setRoleAdmin(bytes32 roleId, bytes32 adminRoleId) public {