Bug 1196631 - Make JS::ubi::Node::size return 1 by default. r=sfink

The old default was zero, but one is a better default in a couple ways: (1)
there are no zero-sized things in C++ so one is closer to the truth, and (2) it
gives us an idea of what magnitude of space is consumed by node specializations
which don't have a JS::ubi::Node::size override yet.

js/public/UbiNode.h

@@ -495,8 +495,8 @@ class Base {
     // Return the size of this node, in bytes. Include any structures that this
     // node owns exclusively that are not exposed as their own ubi::Nodes.
     // |mallocSizeOf| should be a malloc block sizing function; see
-    // |mfbt/MemoryReporting.h.
-    virtual size_t size(mozilla::MallocSizeOf mallocSizeof) const { return 0; }
+    // |mfbt/MemoryReporting.h|.
+    virtual size_t size(mozilla::MallocSizeOf mallocSizeof) const { return 1; }
 
     // Return an EdgeRange that initially contains all the referent's outgoing
     // edges. The caller takes ownership of the EdgeRange.