Root ClientRect wrappers stored in ClientRectList

[jdm]

The client rects are not attached to the DOM tree in any way, nor are they sitting on the stack and thus visible to the conservative stack svanner, so they're sitting ducks for being GCed at any time. A trace hook would be nicer from a conceptual standpoint, but explicit rooting seems like it would be much easier to integrate since it wouldn't have to interact with the autogenerated bindings.

[bzbarsky]

Note that client rects can exist outside a ClientRectList, in case that matters.

[jdm]

We have trace hooks generated by the compiler that do this now. Oh joyous day!