Following doc.openerp.com/trunk/web/module/ the below Code will works:-

web_example/web_example.xml

<openerp>
    <data>
        <record model="ir.actions.client" id="action_client_example">
            <field name="name">Example Client Action</field>
            <field name="tag">example.action</field>
        </record>
        <menuitem action="action_client_example"
                  id="menu_client_example"/>
    </data>
</openerp>

static/src/js/first_module.js

openerp.web_example = function (instance) {
  instance.web.client_actions.add('example.action', 'instance.web_example.Action');
instance.web_example.Action = instance.web.Widget.extend({
    className: 'oe_web_example',
    start: function () {
         this.$el.text("Hello, world!");
        //console.log("Executed the action", action); Action is not defined
        return this._super();
    }
});
};