MD User Manager: Update <style is="custom-style"> for Polymer 2

Wrapping <style is="custom-style"> with <custom-style> per Polymer 2
migration guide.

This also fixes a styling issue with the remove user button, which
occurred when Polymer 2 was turned on by default.

Bug: 896583, 739532
Change-Id: Ic7648996881b945cad2895e675c45afcb095daf0
Reviewed-on: https://chromium-review.googlesource.com/c/1294593Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602046}

ui/login/account_picker/md_user_pod_template.html

@@ -4,7 +4,10 @@
   </template>
 </dom-module>
 
-<style is="custom-style" include="user-pod-template-shared-styles"></style>
+<custom-style>
+  <style is="custom-style" include="user-pod-template-shared-styles"></style>
+</custom-style>
+
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 
 <iron-iconset-svg name="user-pod" size="24">