[DevTools] Make DevToolsWindow load state changes idempotent.

This fixes possible test bugs when can_dock is false.

BUG=none

Review URL: https://codereview.chromium.org/133383003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246021 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246021