DevTools: [CodeMirror] remove auto-inserted whitespaces

Currently, text editor auto-inserts spaces/tabs in certain cases. For example,
if you hit "enter" when your cursor is located on indented line, the new line
will be auto-indented. If you hit "enter" one more time, a new line will be
added, but the previous empty line with trailing auto-added whitespaces will
persist.

This patch suggests removing auto-added indents on every next "enter" keypress.
Note, that they will be removed only if they have not been edited.

R=vsevik, apavlov

Review URL: https://codereview.chromium.org/402433007

git-svn-id: svn://svn.chromium.org/blink/trunk@178515 bbb929c8-8fbe-4397-9dbb-9b2b20218538