MD WebUI: disable custom properties

They were never on to begin with and now that we've reached a new enough
version of Polymer that this flag actually has an effect, it's breaking
things.

TBR=michaelpg@chromium.org
BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation
NOTRY=true

Review-Url: https://codereview.chromium.org/2181073002
Cr-Commit-Position: refs/heads/master@{#407601}

ui/webui/resources/js/polymer_config.js

@@ -6,7 +6,8 @@ if (typeof Polymer == 'undefined') {
   Polymer = {
     dom: 'shadow',
     lazyRegister: true,
-    useNativeCSSProperties: true,
+    // TODO(dbeam): re-enable when this doesn't break things.
+    // useNativeCSSProperties: true,
   };
 } else {
   console.error('Polymer is already defined.');