ContentEditableField should always use DOMSubtreeModified on FF

[Hochhaus, Andrew]

Currently, HAS_DOM_SUBTREE_MODIFIED_EVENT is only true on FF when
USE_CONTENTEDITABLE_IN_FIREFOX_3 is set at compile time. This makes
sense for Field/SeamlessField but on FF ContentEditableField should
always use DOMSubtreeModified in Field#setupMutationEventHandlersGecko
otherwise DOM modifications outside of the editor result in calls to
handleMutationEventGecko_ which can be unnecessarily expensive.

Patch:

https://codereview.appspot.com/6826076

-Andy

[Garry Boyer]

There will be lots of places we missed this. ContentEditableField is a newcomer and wasn't tested with very many plugins :-) If you can -- it'd be very helpful to just do an audit.