Bug 1034239 - Add Proxies to ForceCOWBehavior. r=gabor
Bug 1034239 - Replace GentlyOpaque with an "Xray-to-nothing" wrapper. r=gabor

I did this wrong before. Making this a SecurityWrapper means that the caller does
not subsumes the target, and that the target therefore needs to be protected
from the caller. But GentlyOpaque was supposed to be an analog of PermissiveXray
for use when no useful XrayTraits exist, so it should behave similarly.

If we make this a Filtering Security Wrapper, we get a bunch of assertions where we
expect CheckedUnwrap to succeed for a chrome-side wrapper. And we can't making it
a Filtering Non-Security Wrapper, because then the filtering policy isn't even
consulted (an optimization in jsproxy.cpp).

Really, we want all of the Xray machinery (like the ability to waive and to place
expandos), and we just don't want to resolve any properties. This patch does this.