DevTools: [Styles] Show user/user-agent styles in the computed trace at all times

Sections are created regardless of the "Show user agent styles" setting. They are
just not added to the UI but can be used when building the computed trace.

R=vsevik
BUG=276293

Review URL: https://codereview.chromium.org/23648007

git-svn-id: svn://svn.chromium.org/blink/trunk@158638 bbb929c8-8fbe-4397-9dbb-9b2b20218538