DevTools: [CodeMirror] support multiple selection in BlockIndentController

The patch fixes the code of BlockIndentController to support multiple
selections.

BUG=388640

Review URL: https://codereview.chromium.org/353423002

git-svn-id: svn://svn.chromium.org/blink/trunk@177513 bbb929c8-8fbe-4397-9dbb-9b2b20218538