Re: [matrix][cssom-view] DOMPoint, DOMPointLiteral definitions

I think the cleanest way (at least conceptually*) to support both mutable
and immutable rects and also share interface code would be to have three
interfaces:
-- DOMRect is the base interface for both immutable and mutable rects. It
defines "current x/y/width/height" state and can expose methods that use
that state. Other APIs can take DOMRects and be defined in terms of the
current state. It allows read-only access to the current state and does not
offer any mutation APIs.
-- DOMRectImmutable extends DOMRect and adds the invariant that the state
never changes. It probably doesn't need to add any new API. (However, being
immutable is potentially meaningful to clients, because it allows them to
avoid copying, so testing whether a rect is immutable could have value.)
-- DOMRectMutable extends DOMRect and adds APIs to mutate the rect.

If we were to just do this directly, the main impact on developers would be
that to directly construct a mutable rect they'd have to write "new
DOMRectMutable()" which is a little bit more verbose. Could we make "new
DOMRect()" return a DOMRectMutable? I think we could at the implementation
level, but it might be too magical.

Another approach would be to name the three interfaces DOMRectBase,
DOMRectImmutable, and DOMRect. Then "new DOMRect()" gives you a mutable
rect without magic, but OTOH DOMRectImmutable doesn't extend DOMRect, which
may be surprising.

Or we could just take the above directly and have developers write "new
DOMRectMutable". I actually prefer this option. I think we should not be
adding surprise to the platform just to save a little typing.

Rob

* I.e. this is the way Jeannette Wing would have told me to do it :-).
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