raii-forwarding

Demonstrates how to forward a RAII helper class across a wrapper that doesn't expose the definition of the wrapped class

In this example:

• AsyncPanZoomController is the wrapped class
• APZSampler is the wrapper class
• AutoApplyTestAttributesInternal is the RAII helper class
• AutoApplyTestAttributes is a wrapper for the RAII helper class
• AsyncCompositionManager.cpp is the client code that doesn't have access to the definition of AsyncPanZoomController

The key insights are:

• AutoApplyTestAttributesInternal cannot be nested inside AsyncPanZoomController, because nested classes cannot be forward-declared, and we need APZSampler.h to only forward-declare AutoApplyTestAttributesInternal.
• The destructor of AutoApplyTestAttributes needs to be defined out of line. This ensures that the destructor of unique_ptr<AutoApplyTestAttributesInternal>, which requires AutoApplyTestAttributesInternal to be complete, is instantiated only in APZSampler.cpp and not in AsyncCompositionManager.cpp. (This is also why we need the wrapper AutoApplyTestAttributes in the first place, and can't just have the client code use unique_ptr<AutoApplyTestAttributesInternal> directly; there would be no way to avoid the client code trying to instantiate the destructor of unique_ptr<AutoApplyTestAttributesInternal> in AsyncCompositionManager.cpp.)