Define getSelection on DocumentOrShadowRoot instead of Document

[pmdartus]

Currently, the Selection API spec defines the getSelection via an extension to the Document interface. In the mean time, the Shadow DOM spec defines the getSelection via an exentsion to the DocumentOrShadowRoot interface. Does it makes more sense to define getSelection in the Selection API spec via a DocumentOrShadowRoot extension instead of a Document extension?

Note: There is already a WPT tests in place to test getSelection on ShadowRoot:

• https://github.com/web-platform-tests/wpt/blob/master/shadow-dom/untriaged/elements-and-dom-objects/shadowroot-object/shadowroot-methods/test-004.html
• https://github.com/web-platform-tests/wpt/blob/master/shadow-dom/untriaged/user-interaction/ranges-and-selections/test-002.html

[rniwa]

See the current consensus at WICG/webcomponents#79. The plan is not add getSelection but rather add getComposedRange to Selection interface.

[rniwa]

Closing in favor of the issue #98