Bug 1387381 - Remove nsXPIDLString local variables. r=erahm.

nsXPIDLStrings are marked as VOIDED upon initialization. Most of these local
nsXPIDLString variables are immediately set via getter_Copies(), which will
either assign a string value (using Adopt()) or do SetIsVoid(). These can be
trivially converted to nsString, which will get the same treatment.

The patch suitably converts the remaining nsXPIDLString local variable as well.

docshell/base/nsDocShell.cpp

@@ -1305,7 +1305,7 @@ nsDocShell::LoadURI(nsIURI* aURI,
   uint32_t referrerPolicy = mozilla::net::RP_Unset;
   bool isSrcdoc = false;
   nsCOMPtr<nsISHEntry> shEntry;
-  nsXPIDLString target;
+  nsString target;
   nsAutoString srcdoc;
   nsCOMPtr<nsIDocShell> sourceDocShell;
   nsCOMPtr<nsIURI> baseURI;
@@ -4288,7 +4288,6 @@ nsDocShell::FindChildWithName(const nsAString& aName,
     return NS_OK;
   }
 
-  nsXPIDLString childName;
   nsTObserverArray<nsDocLoader*>::ForwardIterator iter(mChildList);
   while (iter.HasMore()) {
     nsCOMPtr<nsIDocShellTreeItem> child = do_QueryObject(iter.GetNext());