hide GH login in mobile view

Author: DanielRyanSmith

webapp/components/wpt-header.js

@@ -195,7 +195,9 @@ class WPTHeader extends WPTFlags(PolymerElement) {
         <h1>
       </div>
       <template is="dom-if" if="[[githubLogin]]">
-        <github-login user="[[user]]" is-triage-mode="[[isTriageMode]]"></github-login>
+        <div id="desktop-login">
+          <github-login user="[[user]]" is-triage-mode="[[isTriageMode]]"></github-login>
+        </div>
       </template>
       <button
           class$="[[_computeMenuButtonClass(_isMenuOpen)]]"