DevTools: fixed layers panel update upon navigation

Fixed several problems that prevented Layers panel from updating
properly during navigation. Also, fixed layer properties update
upon layer tree update.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23523053

git-svn-id: svn://svn.chromium.org/blink/trunk@157748 bbb929c8-8fbe-4397-9dbb-9b2b20218538