DevTools Paint Profiler: display progress banner while profiling snapshot

- clear command log and duration bars while loading new snapshot;
- display "Profiling..." in overview bars window while profiling the snapshot.

BUG=

Review URL: https://codereview.chromium.org/583953002

git-svn-id: svn://svn.chromium.org/blink/trunk@182400 bbb929c8-8fbe-4397-9dbb-9b2b20218538