Call style directly in InspectorCSSAgent::asInspectorStyleSheet

We can just call style() directly here, internally it does the same check.

Review URL: https://codereview.chromium.org/442473002

git-svn-id: svn://svn.chromium.org/blink/trunk@179516 bbb929c8-8fbe-4397-9dbb-9b2b20218538