Rename nsIDocument to mozilla::dom::Document.

Summary: Really sorry for the size of the patch. It's mostly automatic
s/nsIDocument/Document/ but I had to fix up in a bunch of places manually to
add the right namespacing and such.

Overall it's not a very interesting patch I think.

nsDocument.cpp turns into Document.cpp, nsIDocument.h into Document.h and
nsIDocumentInlines.h into DocumentInlines.h.

I also changed a bunch of nsCOMPtr usage to RefPtr, but not all of it.

While fixing up some of the bits I also removed some unneeded OwnerDoc() null
checks and such, but I didn't do anything riskier than that.

Bug: 1517241
Reviewed-by: smaug

components/style/gecko/media_queries.rs

@@ -162,8 +162,8 @@ impl Device {
 
     /// Gets the document pointer.
     #[inline]
-    pub fn document(&self) -> *mut structs::nsIDocument {
-        self.pres_context().mDocument.raw::<structs::nsIDocument>()
+    pub fn document(&self) -> *mut structs::Document {
+        self.pres_context().mDocument.mRawPtr
     }
 
     /// Recreates the default computed values.