Bug 1525720, part 13 - Stop inheriting nsIRemoteTab interface in BrowserParent. r=nika

This commit removes nsIRemoteTab as a parent class from BrowserParent,
so that BrowserHost is the only concrete implementation of nsIRemoteTab.

Some static_cast's are updated to cast to BrowserHost, and other places
have to be updated to pass a BrowserHost instead of a BrowserParent.

WindowGlobalParent had a getter to return it's managing BrowserParent
as a nsIRemoteTab. I couldn't find a use of this in-tree, so I've just
opt-ed to remove it. If there's a use-case, we can add something back
in.

Differential Revision: https://phabricator.services.mozilla.com/D31444

accessible/generic/RootAccessible.cpp

@@ -27,6 +27,7 @@
 #include "mozilla/dom/CustomEvent.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/dom/BrowserHost.h"
 
 #include "nsIDocShellTreeItem.h"
 #include "nsIDocShellTreeOwner.h"
@@ -669,12 +670,12 @@ ProxyAccessible* RootAccessible::GetPrimaryRemoteTopLevelContentDoc() const {
   mDocumentNode->GetDocShell()->GetTreeOwner(getter_AddRefs(owner));
   NS_ENSURE_TRUE(owner, nullptr);
 
-  nsCOMPtr<nsIRemoteTab> browserParent;
-  owner->GetPrimaryRemoteTab(getter_AddRefs(browserParent));
-  if (!browserParent) {
+  nsCOMPtr<nsIRemoteTab> remoteTab;
+  owner->GetPrimaryRemoteTab(getter_AddRefs(remoteTab));
+  if (!remoteTab) {
     return nullptr;
   }
 
-  auto tab = static_cast<dom::BrowserParent*>(browserParent.get());
+  auto tab = static_cast<dom::BrowserHost*>(remoteTab.get());
   return tab->GetTopLevelDocAccessible();
 }