Disable stacked panel mode for Linux-aura

We do not have the implementation for stacked panel support
on Linux-aura. Disbale it because otherwise it will cause crash.

BUG=326132
TBR=benwells@chromium.org

Review URL: https://codereview.chromium.org/189503003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256118 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256118