Bug 843231 - Move the nulling out of mPrototypeNoHelper from SetGlobal to RemoveWrappedNativeProtos. r=bz

The only call-site for both of these is JSD->nsXPConnect::InitClasses, so this
is equivalent. All this will go away soon anyhow when JSD goes away.
Bug 843231 - Remove SetGlobal. r=bz

This stuff has all been simplified significantly with CPG, and this is no longer
necessary. We always have a global when we construct a scope, and it never
changes.
Bug 843231 - Alter the XBL scope test to make it compatible with reading the dom.xbl_scopes only once per global. r=bz

--HG--
rename : content/xbl/test/test_bug821850.xhtml => content/xbl/test/file_bug821850.xhtml
Bug 843231 - Compute whether to use an XBL scope exactly once. r=bz