Make isEditablePosition to work with Position objects other than PositionIsOffsetInAnchor

This patch makes isEditablePosition to work with Position objects other than
PositionIsOffsetInAnchor by using Position::parentAnchoredEquivalent() to
use right Node object for checking editability.

BUG=348973
TEST=LayoutTests/editing/execCommand/format-block-with-uneditable-crash.html

Review URL: https://codereview.chromium.org/309993002

git-svn-id: svn://svn.chromium.org/blink/trunk@175660 bbb929c8-8fbe-4397-9dbb-9b2b20218538