DevTools: simpleSelector should not contain nth-child specificity

This patch fixes DOMPresentationUtils.simpleSelector method to not
include nth-child attribute.

BUG=349089

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168803

Review URL: https://codereview.chromium.org/183683023

git-svn-id: svn://svn.chromium.org/blink/trunk@169348 bbb929c8-8fbe-4397-9dbb-9b2b20218538