DevTools: Remove style sheet list from activeStyleSheetsUpdated instrumentation and allow inspector to request active style sheets when it needs them.

This will allow us to query all the active style sheets at the inspector start time and to avoid collecting all active style sheets when inspector does not really need them.

Review URL: https://codereview.chromium.org/116083005

git-svn-id: svn://svn.chromium.org/blink/trunk@164339 bbb929c8-8fbe-4397-9dbb-9b2b20218538