Avoid to use abbreviated names in {first,last}EditableVisiblePosition{After,Before}PositionInRoot

This patch renames abbreviated name "p" to "editablePosition" in {first,last}EditableVisiblePosition{After,Before}PositionInRoot for improving readability and ease of maintenance.

This patch is a preparation of introducing |lastEditablePositionBeforePositionInRoot.

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/342553008

git-svn-id: svn://svn.chromium.org/blink/trunk@176491 bbb929c8-8fbe-4397-9dbb-9b2b20218538