ReplaceSelectionCommand::makeInsertedContentRoundTrippableWithHTMLTreeBuilder should take a const reference to InsertedNodes

This method doesn't modify InsertedNodes object so that it can take a const reference.

BUG=

Review URL: https://codereview.chromium.org/200613003

git-svn-id: svn://svn.chromium.org/blink/trunk@169431 bbb929c8-8fbe-4397-9dbb-9b2b20218538