Bug 1547911 - Don't assert that there are no children when restoring BC. r=nika

This is essentially equal to restoring cached children and removing
current children from a BrowsingContext, which is the correct
behaviour. It would've been better if the current children were
removed in a more transparent manner, but it is more important to
remove an assert that too eagerly triggers.

Differential Revision: https://phabricator.services.mozilla.com/D29945

--HG--
extra : moz-landing-system : lando

docshell/base/BrowsingContext.cpp

@@ -339,8 +339,6 @@ void BrowsingContext::RestoreChildren(Children&& aChildren, bool aFromIPC) {
           ("%s: Restoring children of 0x%08" PRIx64 "",
            XRE_IsParentProcess() ? "Parent" : "Child", Id()));
 
-  MOZ_DIAGNOSTIC_ASSERT(mChildren.IsEmpty());
-
   for (BrowsingContext* child : aChildren) {
     MOZ_DIAGNOSTIC_ASSERT(child->GetParent() == this);
     Unused << Group()->EvictCachedContext(child);