DevTools: [SSP] do not rebuild styles when sidebar pane is not showing

BUG=425523
R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/666163003

git-svn-id: svn://svn.chromium.org/blink/trunk@184162 bbb929c8-8fbe-4397-9dbb-9b2b20218538