Make setBaseAndExtent and other setter methods on Selection work with shadow DOM

[rniwa]

Per WICG/webcomponents#79, we should make setBaseAndExtent and other methods work with shadow DOM. Namely, when a node inside a shadow tree is given to setBaseAndExtent, it shouldn't throw or adjust the end points to be in the same tree.

[rniwa]

At least collapse working with shadow DOM is a web compatibility requirement at this point:
WebKit/WebKit@c48d719

[annevk]

Hmm, so when a shadow tree child is selected it's exposed through a global API? That seems rather bad.

[rniwa]

No, it's not exposed. It's just that these setter functions need to allow referencing into nodes within shadow trees. Getters will continue to return the outer most host.