DevTools: [CSSStyleModel] Do not send multiple computed style requests when requested synchronously

R=pfeldman, vsevik
BUG=295643

Review URL: https://codereview.chromium.org/23902052

git-svn-id: svn://svn.chromium.org/blink/trunk@158923 bbb929c8-8fbe-4397-9dbb-9b2b20218538