DevTools: add test to verify matched styles for :host selector

This patch adds test based on the regression issue with styles sidebar
pane not showing styles for :host selectors. This test was originally
added in r174963, but the commit got reverted.

BUG=363468
TBR=apavlov

Review URL: https://codereview.chromium.org/317703002

git-svn-id: svn://svn.chromium.org/blink/trunk@175553 bbb929c8-8fbe-4397-9dbb-9b2b20218538