DevTools: lock panel switching when timeline is active.

Drive by: fix exception in Counters code.

BUG=381115

Review URL: https://codereview.chromium.org/345803003

git-svn-id: svn://svn.chromium.org/blink/trunk@176855 bbb929c8-8fbe-4397-9dbb-9b2b20218538