This causes e10s to behave differently from non e10s, because in e10s we don't
forward the creation of the document to the parent process. However this only
seems to happen in strange edge cases.
We should never create DocAccessibles for documents that are going away so it
seems like this shouldn't be necessary, but without a test case its hard to
know why we are creating DocAccessibles for documents without docshells. So
for now work around the issue and hope it doesn't matter in practice.
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.
