Use simple CSS selectors to identify elements in Layer's panel tree view

Drive-by: fix navigation to Elements panel via context menu.

BUG=

Review URL: https://codereview.chromium.org/206123005

git-svn-id: svn://svn.chromium.org/blink/trunk@169662 bbb929c8-8fbe-4397-9dbb-9b2b20218538