Don't attempt to session restore an undocked devtools window in AURA.

Attempting to restore this window does not work anyway as it does not have the associated web contents wired up.
This is the same behavior as on non AURA.

Fixes bug https://code.google.com/p/chromium/issues/detail?id=304138

BUG=304138
R=sky@chromium.org, sky

Review URL: https://codereview.chromium.org/51943007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232838 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232838