DevTools: added "overlayContents" mode, where DevTools content is placed around and underneath insepcted contents.

BUG=318751,304730

Review URL: https://codereview.chromium.org/71633003

git-svn-id: svn://svn.chromium.org/blink/trunk@163695 bbb929c8-8fbe-4397-9dbb-9b2b20218538