Verify DOM.enable() exists before calling it

This is for #10191 

Targets `release` branch.

is `Inspector.DOM.enable()` a new function? We weren't calling it prior to #10088 so I'm wondering if we should be calling it and should it be written as 
```javascript
var domEnablePromise = Inspector.DOM.enable ? `Inspector.DOM.enable() :  (new $.Deferred()).resolve().promise();
```