Prevent crash in WebView when disabling the hw renderer

Bug #6596807

A crash would occur in the following situation:
- WebView registers a functor with the hardware renderer
- The hardware renderer gets disabled
- WebView attemps to unregister its functor

Unregistering the functor fails because the hardware renderer is now disabled.
When the renderer becomes enabled again, the functor is invoked, which leads
to a native crash.

This change simply allows functors to always be unregistered, even when the
renderer is disabled. A disabled renderer only means that it will not be used
for rendering; as such, unregistering a functor is a valid operation and
should be allowed.

Change-Id: I0ff897a0cca7e048c609033215cd0f7f5c940bcc