DevTools: disable layer tree agent when layers panel is not shown

We don't keep any state in layer tree agent and enabling/disabling
it is free, so to keep instrumentation overhead and protocol traffic
down, just disable agent when the panel is not shown.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/25686004

git-svn-id: svn://svn.chromium.org/blink/trunk@158803 bbb929c8-8fbe-4397-9dbb-9b2b20218538