Roll Polymer from 1.5.0 -> 1.6.0 to pick up native CSS custom props

third_party/polymer/v1_0/components-chromium/paper-dropdown-menu/paper-dropdown-menu.html

Index: third_party/polymer/v1_0/components-chromium/paper-dropdown-menu/paper-dropdown-menu.html
diff --git a/third_party/polymer/v1_0/components-chromium/paper-dropdown-menu/paper-dropdown-menu.html b/third_party/polymer/v1_0/components-chromium/paper-dropdown-menu/paper-dropdown-menu.html
index e9cd0b4f48573c145af0a2d8fd2ac0b28a054a6f..091dc4200534e098837c6e5598c2646f20985361 100644
--- a/third_party/polymer/v1_0/components-chromium/paper-dropdown-menu/paper-dropdown-menu.html
+++ b/third_party/polymer/v1_0/components-chromium/paper-dropdown-menu/paper-dropdown-menu.html
@@ -83,8 +83,8 @@ respectively.
     <style include="paper-dropdown-menu-shared-styles"></style>
 
     <!-- this div fulfills an a11y requirement for combobox, do not remove -->
-    <div role="button"></div>
-    <paper-menu-button id="menuButton" vertical-align="[[verticalAlign]]" horizontal-align="[[horizontalAlign]]" vertical-offset="[[_computeMenuVerticalOffset(noLabelFloat)]]" disabled="[[disabled]]" no-animations="[[noAnimations]]" on-iron-select="_onIronSelect" on-iron-deselect="_onIronDeselect" opened="{{opened}}">
+    <span role="button"></span>
+    <paper-menu-button id="menuButton" vertical-align="[[verticalAlign]]" horizontal-align="[[horizontalAlign]]" vertical-offset="[[_computeMenuVerticalOffset(noLabelFloat)]]" disabled="[[disabled]]" no-animations="[[noAnimations]]" on-iron-select="_onIronSelect" on-iron-deselect="_onIronDeselect" opened="{{opened}}" close-on-activate="" allow-outside-scroll="[[allowOutsideScroll]]">
       <div class="dropdown-trigger">
         <paper-ripple></paper-ripple>
         <!-- paper-input has type="text" for a11y, do not remove -->