developer_mode/views/web_view.xml

<openerp>
  <data>
      <record model="ir.ui.view" id="developer_mode_active_form">
          <field name="name">DeveloperMode</field>
          <field name="model"></field>
          <field name="inherit_id" ref="web.login"/>
          <field name="arch" type="xml">
              <xpath expr="//input[@name='redirect']" position='attributes'>
                  <attribute name="t-att-value">'%s%sdebug=1' % (redirect or '/web', redirect and '?' in redirect and '&amp;' or '?')</attribute>
              </xpath>
          </field>
      </record>
  </data>
</openerp>