DevTools: misc layer panel polish

- fix 3d layer view rotation;
- deselect layer upon click outside of layer area in 3d view;
- show empty view with a hint message if not in composited mode;

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/22859051

git-svn-id: svn://svn.chromium.org/blink/trunk@156626 bbb929c8-8fbe-4397-9dbb-9b2b20218538