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
diff --git a/ui/webui/resources/cr_elements/policy/README.md b/ui/webui/resources/cr_elements/policy/README.md
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+Policy indicators show when a setting is controlled by something other than the
+user, especially user/device policy. The badge icons are sourced from
+[cr_elements/icons.html](../icons.html) by default. Indicators show a tooltip on
+hover when `CrPolicyStrings` is defined.
+
+Elements like `<cr-policy-pref-indicator>` and `<cr-policy-network-indicator>`
+are designed to be reused in WebUI pages:
+
+    <cr-policy-pref-indicator pref="[[pref]]"></cr-policy-pref-indicator>
+
+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.