DevTools: speculative fix for CSS inspection code crash.

The patch adds a check that will eliminate any possibility of trying to
populate properties for non-StyleRule rules.

BUG=397300

Review URL: https://codereview.chromium.org/436233003

git-svn-id: svn://svn.chromium.org/blink/trunk@179591 bbb929c8-8fbe-4397-9dbb-9b2b20218538