Revert of Remove unused sourceData variable in InspectorStyleSheet. (patchset #1 id:1 of https://codereview.chromium.org/574963002/)

Reason for revert:
This code is still needed

Original issue's description:
> Remove unused sourceData variable in InspectorStyleSheet.
> 
> Breaks build for targets that treat unused variables as errors.
> 
> R=dgozman@chromium.org, vsevik@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182145

TBR=dgozman@chromium.org,ager@chromium.org

Review URL: https://codereview.chromium.org/611333002

git-svn-id: svn://svn.chromium.org/blink/trunk@182928 bbb929c8-8fbe-4397-9dbb-9b2b20218538