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
+
+Setting that can't be controlled by the current user often show an icon and a

[dschuyler, 2017/02/28 22:16:33]

maybe
s/Setting/Settings/

or 
s/Setting/A setting/

[michaelpg, 2017/03/01 00:34:00]

Done.

+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, assuming
+`CrPolicyStrings` is defined.

[dschuyler, 2017/02/28 22:16:33]

maybe s/assuming/assuming an entry in/
(since CrPolicyStrings will be defined unconditionally iiuc).

[michaelpg, 2017/03/01 00:34:00]

No, the actual object needs to be defined -- we do this in settings_ui.js and
some tests.

If it's not defined we just don't display tooltips. I think OOBE is like that.

+
+## 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-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