DevTools: [SSP] update SSP when css model gets enabled.

There was a typo in the code responsible for the styles sidebar pane
update whenever the css model gets enabled. The patch fixes the typo.

BUG=400000
TBR=pfeldman
R=vsevik

Review URL: https://codereview.chromium.org/495933002

git-svn-id: svn://svn.chromium.org/blink/trunk@180725 bbb929c8-8fbe-4397-9dbb-9b2b20218538