Have ReplaceSelectionCommand::InsertedNodes API take references in argument

Have ReplaceSelectionCommand::InsertedNodes API take references in argument.
This makes the code look safer and gets rid of several unnecessary null
checks.

R=adamk

Review URL: https://codereview.chromium.org/54363006

git-svn-id: svn://svn.chromium.org/blink/trunk@161257 bbb929c8-8fbe-4397-9dbb-9b2b20218538