Remove unused sourceData variable in InspectorStyleSheet.

Breaks build for targets that treat unused variables as errors.

R=dgozman@chromium.org, vsevik@chromium.org

Review URL: https://codereview.chromium.org/574963002

git-svn-id: svn://svn.chromium.org/blink/trunk@182145 bbb929c8-8fbe-4397-9dbb-9b2b20218538