Get rid of redundant functions Editor::indent and outdent

This patch removes redundant functions Editor::indent() and outdent(), which were used for special handling of WebFrame::executeCommand.

Editor::indent/outdent are identical to executeIndent/executeOutend.

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

Review URL: https://codereview.chromium.org/356173002

git-svn-id: svn://svn.chromium.org/blink/trunk@177073 bbb929c8-8fbe-4397-9dbb-9b2b20218538