Add explicit width and height to crown logo.

PageSpeed suggests: "Specifying a width and height for all images allows for
faster rendering by eliminating the need for unnecessary reflows and
repaints."

We never seem to resize this image, so this shouldn't do any harm.

source/views/layouts/govuk_template.html.erb

@@ -71,7 +71,7 @@
         <div class="header-global">
           <div class="header-logo">
             <a href="https://www.gov.uk/" title="Go to the GOV.UK homepage" id="logo" class="content">
-              <img src="<%= asset_path 'gov.uk_logotype_crown.png' %>" alt=""> GOV.UK
+              <img src="<%= asset_path 'gov.uk_logotype_crown.png' %>" width="35" height="31" alt=""> GOV.UK
             </a>
           </div>
           <%= yield :inside_header %>