Update Polymer components.

third_party/polymer/components/iron-dropdown/.bower.json

Index: third_party/polymer/components/iron-dropdown/.bower.json
diff --git a/third_party/polymer/components/iron-dropdown/.bower.json b/third_party/polymer/components/iron-dropdown/.bower.json
index 93a6219fabdd762d7f09eb9eda513d55d822befd..0f0841b4da59d39942157d8594d74de2a27746f9 100644
--- a/third_party/polymer/components/iron-dropdown/.bower.json
+++ b/third_party/polymer/components/iron-dropdown/.bower.json
@@ -1,6 +1,6 @@
 {
   "name": "iron-dropdown",
-  "version": "1.5.5",
+  "version": "1.5.6",
   "description": "An unstyled element that works similarly to a native browser select",
   "authors": [
     "The Polymer Authors"
@@ -35,11 +35,11 @@
     "iron-image": "polymerelements/iron-image#^1.0.0"
   },
   "ignore": [],
-  "_release": "1.5.5",
+  "_release": "1.5.6",
   "_resolution": {
     "type": "version",
-    "tag": "v1.5.5",
-    "commit": "6d379de1076062ff258e69064211261d14599b0b"
+    "tag": "v1.5.6",
+    "commit": "3507dcb983bc9d8dc9ad181daacfe0f4f8b97415"
   },
   "_source": "https://github.com/PolymerElements/iron-dropdown.git",
   "_target": "^1.0.0",