Bug 1525720, part 4 - Assert in DrawSnapshot if not the chrome process. r=mattwoodrow

This API is only intended to be used in the chrome process and this commit
makes this explicit to simplify a later refactoring.

Differential Revision: https://phabricator.services.mozilla.com/D31434

--HG--
extra : rebase_source : 79f143fb7fce889284b23fb65580f8e8dab83df7
extra : histedit_source : 823209f777fcb0e3e6579ac1afc0d6bf8e566051

dom/base/nsFrameLoader.cpp

@@ -3235,6 +3235,12 @@ void nsFrameLoader::Print(uint64_t aOuterWindowID,
 already_AddRefed<mozilla::dom::Promise> nsFrameLoader::DrawSnapshot(
     double aX, double aY, double aW, double aH, double aScale,
     const nsAString& aBackgroundColor, mozilla::ErrorResult& aRv) {
+  MOZ_ASSERT(XRE_IsParentProcess());
+  if (!XRE_IsParentProcess()) {
+    aRv = NS_ERROR_FAILURE;
+    return nullptr;
+  }
+
   RefPtr<nsIGlobalObject> global = GetOwnerContent()->GetOwnerGlobal();
   RefPtr<Promise> promise = Promise::Create(global, aRv);
   if (NS_WARN_IF(aRv.Failed())) {