Create a new definition to change the header content:
<definition name="baseLayout" template="/WEB-INF/jsp/layout/layout.jsp">
<!-- Defined here -->
<put-attribute name="head" value="" />
<put-attribute name="title" value="" />
...
</definition>
<definition name="videos" extends="baseLayout">
<!-- Overridden here -->
<put-attribute name="head" value="/WEB-INF/jsp/videos-additionalHeadDefinitions.jsp" />
...
</definition>
And add this to you layout.jsp:
<head>
<!-- This is content from the layout.jsp file -->
<title>Welcome</title>
...
<tiles:insertAttribute name="head" />
</head>