Use profiling lock in ScreencastView to detect if Timeline or Profiler is started

Instead of listening to particular model notifications it is enough to listen for profiling lock changes.

BUG=403684
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/471013002

git-svn-id: svn://svn.chromium.org/blink/trunk@180352 bbb929c8-8fbe-4397-9dbb-9b2b20218538