Avoid duplicate null check in ReplaceSelectionCommand::pastLastLeaf()

Avoid duplicate null check in ReplaceSelectionCommand::pastLastLeaf() by
calling m_lastNodeInserted->lastDescendant() directly instead of
lastLeafInserted(). This also makes it clearer by we are checking that
m_lastNodeInserted is not null as it is now explicitly use right after.
This avoid avoids dereferencing a pointer as we now get a reference
directly.

R=adamk

Review URL: https://codereview.chromium.org/189913017

git-svn-id: svn://svn.chromium.org/blink/trunk@168848 bbb929c8-8fbe-4397-9dbb-9b2b20218538