bug 1098386 - Allow PDocAccessibleConstructor to be recieved when gAccessibilityService doesn't exist r=surkov

Talos causes accessibility to be instantiated only in the content
process.  That means PDocAccessibleConstructor messages can be recieved
by the parent when GetAccService() returns null.  The easiest way of
dealing with this is moving DocManager::mRemoteDocuments from a member
of the acc service singleton to its own global.  That means the parent
process can track accessible documents in child processes without
instantiating a11y in the parent process.
bug 1110510 - make sure to create and destroy proxies for documents r=surkov