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

third_party/polymer/v1_0/components/paper-checkbox/README.md

 # paper-checkbox
 
 `paper-checkbox` is a button that can be either checked or unchecked.  User
 can tap the checkbox to check or uncheck it.  Usually you use checkboxes
 to allow user to select multiple options from a set.  If you have a single
 ON/OFF option, avoid using a single checkbox and use `paper-toggle-button`
 instead.
 
 Example:
 
 ```html
 <paper-checkbox>label</paper-checkbox>
 
 <paper-checkbox checked>label</paper-checkbox>
 ```
 
 Styling a checkbox:
 
 ```html
 <style is="custom-style">
   paper-checkbox {
+    --paper-checkbox-label-color: #000;
+    --paper-checkbox-checkmark-color: #fff;
+
     /* Unhecked state colors. */
     --paper-checkbox-unchecked-color: #5a5a5a;
+    --paper-checkbox-unchecked-background-color: #5a5a5a;
     --paper-checkbox-unchecked-ink-color: #5a5a5a;
 
     /* Checked state colors. */
     --paper-checkbox-checked-color: #009688;
     --paper-checkbox-checked-ink-color: #009688;
   }
 </style>
 ```