Update bower.json for Polymer elements and add PRESUBMIT.py

third_party/polymer/v1_0/components/paper-radio-button/README.md

 # paper-radio-button
 
 `paper-radio-button` is a button that can be either checked or unchecked.
-User can tap the radio button to check it.  But it cannot be unchecked by
-tapping once checked.
+User can tap the radio button to check or uncheck it.
 
-Use `paper-radio-group` to group a set of radio buttons. When radio buttons
-are inside a radio group, only one radio button in the group can be checked.
-
+Use a `<paper-radio-group>` to group a set of radio buttons.  When radio buttons
+are inside a radio group, exactly one radio button in the group can be checked
+at any time.
 Example:
 
 ```html
 <paper-radio-button></paper-radio-button>
+<paper-radio-button>Item label</paper-radio-button>
 ```
 Styling a radio button:
 
 ```html
 <style is="custom-style">
   :root {
     /* Unchecked state colors. */
     --paper-radio-button-unchecked-color: #5a5a5a;
+    --paper-radio-button-unchecked-background-color: #fff;
     --paper-radio-button-unchecked-ink-color: #5a5a5a;
 
     /* Checked state colors. */
     --paper-radio-button-checked-color: #009688;
     --paper-radio-button-checked-ink-color: #0f9d58;
+
+    --paper-radio-button-label-color: black;
   }
 </style>
 ```