With native Shadow DOM, the selections returned by document.getSelection()
cannot reference the elements actually making up the diff because those
live in the Shadow DOM of gr-diff. Instead, a shadowRoot.getSelection()
method is offered that works as the pre-Shadow DOM Selection used to,
but that one is only available from gr-diff.
This change moves listening for the selectionchange event and retrieving
the selection to gr-diff, which then delegates the extraction and
normalization of the ranges to gr-diff-highlight (it's current
location).
This change also fixes an incompatibility with native Shadow DOM in
gr-selection-action-box, where a call to parentElement cannot cross the
Shadow DOM boundary and needs special handling to retrieve the Shadow
Host.
Together these changes fix creating range comments for native Shadow
DOM.
Bug: Issue 6372
Change-Id: I4bce21025ab5a4c06d77db096d6aa56e7840987b
201610028d