DevTools: Fix edit-as-html behaviour

This patch fixes the wierd behaviour which happens with "Edit as HTML"
editor.

BUG=272410
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23068010

git-svn-id: svn://svn.chromium.org/blink/trunk@156400 bbb929c8-8fbe-4397-9dbb-9b2b20218538