Partially revert r177905

Before r177905, CompositeEditCommand::copyMarkers was doing a deep copy. r177905 changed it to a shallow copy. This is wrong (as the method name suggests), and this CL reverts it back to the deep copy.

BUG=393448
R=yosin@chromium.org

Review URL: https://codereview.chromium.org/391593003

git-svn-id: svn://svn.chromium.org/blink/trunk@178024 bbb929c8-8fbe-4397-9dbb-9b2b20218538