(Spun off https://chromium-review.googlesource.com/c/chromium/src/+/689614)

Right now, the bindings layer supports two related extended attributes:

* [SameObject], which comes from WebIDL itself. As the spec says, "if the [SameObject] extended attribute appears on a read only attribute, then it indicates that when getting the value of the attribute on a given object, the same value must always be returned".
* [SaveSameObject] is Blink-specific and caches the object returned by an attribute getter so that the getter is actually only called once.

Right now, the [SameObject] extended attribute on its own does not do anything at all. I lack the historical context to understand why the two extended attributes are handled separately (even after looking at  bug 462913 ).

I wonder if we could just merge [SaveSameObject] into [SameObject] and make the former's behavior the default when one adds the [SameObject] extended attribute to a readonly attribute declaration.

Read-only attribute declarations are also affected by our binding's layer |is_keep_alive_for_gc| work: in this case, should we prefer [SameObject]'s behavior over is_keep_alive_for_gc's? Is the caching currently done by [SaveSameObject] enough to prevent these objects from being GC'ed by V8?