Update composed tree version of isEndOfEditableOrNonEditableContent() to handle a positions in INPUT/TEXTAREA element

This patch makes composed tree version of
|isEndOfEditableOrNonEditableContent()| to handle a position in INPUT/TEXTAREA
element as same as DOM tree version as a preparation of introducing full version
of |VisibleSelectionTemplate|.

Before this patch, composed tree version of
|isEndOfEditableOrNonEditableContent()| returns false for the last position of
inner editor in INPUT/TEXTAREA element, but DOM version of it returns true.
Because of the last position of inner editor is the last position of
shadow DOM tree and there are no next position. However, in composed tree,
next position of the last position of inner editor is a position after
INPUT/TEXTAREA element.

This patch is a preparation of making selection to handle granularity for web
component, http://crrev.com/1363873004

BUG=513568
TEST=VisibleUnitsTest.isEndOfEditableOrNonEditableContentWithInput

Review URL: https://codereview.chromium.org/1365263002

Cr-Commit-Position: refs/heads/master@{#350803}
2 files changed