Allow features to require optional permissions

Design doc:
https://docs.google.com/document/d/1OhL0Yh7SmWffXyjW_XVQOK95Fqh7gLltk1eEtnKN8Ds/edit

Fixed: twpowertools:86
Change-Id: Iccb22aac2b285307854b7a4c002e9702c24d57f2
diff --git a/src/static/options/experiments.html b/src/static/options/experiments.html
index f5452fa..0f42f11 100644
--- a/src/static/options/experiments.html
+++ b/src/static/options/experiments.html
@@ -12,6 +12,7 @@
       <h1 data-i18n="experiments_title"></h1>
       <p data-i18n="experiments_description"></p>
       <form>
+        <div id="optional-permissions-warning" hidden data-i18n="optionalpermissionswarning_header"></div>
         <div class="actions"><button id="save" data-i18n="save"></button></div>
       </form>
       <div id="save-indicator"></div>