DevTools: [Styles] Improve UI for toggling computed inherited properties display

Drive-by: Make the Computed pane background white (even though the section is read-only)

R=aandrey, eustas, vsevik
BUG=333844

Review URL: https://codereview.chromium.org/130803003

git-svn-id: svn://svn.chromium.org/blink/trunk@164991 bbb929c8-8fbe-4397-9dbb-9b2b20218538