DevTools: make paint profiler target-aware

- introduce WeakReference, that is cleared when its
    host is disposed;
- pass target around as a WeakReference, so that when
    the target is disposed, the caller won't be able to
    use stale object;
- replace direct calls to LayerAgent's paint profiler
    methods with calls to PaintProfilerSnapshot;

Review URL: https://codereview.chromium.org/389563002

git-svn-id: svn://svn.chromium.org/blink/trunk@178040 bbb929c8-8fbe-4397-9dbb-9b2b20218538