Bug 1316683 - Avoid going into SpiderMonkey for retrieving origin attributes; r=baku

Our caller is C++ code, and the implementations are all also written in C++,
so there is no reason to go through SpiderMonkey here.  This patch also makes
nsILoadContext builtinclass to ensure that the implementation is always native.

docshell/base/nsDocShell.cpp

@@ -14944,3 +14944,9 @@ nsDocShell::GetAwaitingLargeAlloc(bool* aResult)
   *aResult = static_cast<TabChild*>(tabChild.get())->IsAwaitingLargeAlloc();
   return NS_OK;
 }
+
+NS_IMETHODIMP_(void)
+nsDocShell::GetOriginAttributes(mozilla::OriginAttributes& aAttrs)
+{
+  aAttrs = mOriginAttributes;
+}