Move common cr-policy-indicator behavior into CrPolicyIndicatorBehavior

ui/webui/resources/cr_elements/policy/README.md

Index: ui/webui/resources/cr_elements/policy/README.md
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+# Policy indicators
+
+Settings that can't be controlled by the current user often show an icon and a
+tooltip explaining why. This happens when a setting is:
+
+* enforced by user policy, or different from a policy's "recommended" value
+* overridden by an extension
+* or (on Chrome OS):
+    * enforced/recommended by device policy (for enrolled devices)
+    * set by the device owner (for non-enrolled devices)
+    * controlled by the primary user (for multiple profile sessions)
+
+## Indicator UI
+
+The badge icons are sourced from [cr_elements/icons.html] by default.
+
+Indicators show a tooltip with explanatory text on hover if `CrPolicyStrings`
+is set; see [settings_ui.js] for an example from MD Settings.
+
+## Using an indicator
+
+Elements like `<cr-policy-pref-indicator>` and `<cr-policy-network-indicator>`
+are provided to be reused in WebUI pages:
+
+    <cr-policy-pref-indicator pref="[[pref]]"></cr-policy-pref-indicator>
+
+Example: [settings-checkbox].
+
+For one-off or composed elements, `CrPolicyIndicatorBehavior` provides some
+configurable properties and calculates dependent properties, such as the
+tooltip, icon, and visibility of the indicator.
+
+Example: [cr_policy_pref_indicator.js] overrides `indicatorType` and
+`indicatorTooltip`. [cr_policy_pref_indicator.html] displays the computed
+properties from `CrPolicyIndicatorBehavior`.
+
+
+[cr_elements/icons.html]: ../icons.html
+[settings_ui.js]: /chrome/browser/resources/settings/settings_ui/settings_ui.js
+[settings-checkbox]: /chrome/browser/resources/settings/controls/settings_checkbox.html
+[cr_policy_pref_indicator.js]: cr_policy_pref_indicator.js
+[cr_policy_pref_indicator.html]: cr_policy_pref_indicator.html