Remove third_party/polymer from .gitignore

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

Index: third_party/polymer/v1_0/components/paper-checkbox/README.md
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-# 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>
-```