DevTools: lock profiler lock when tracing is working.

BUG=381115
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/358563003

git-svn-id: svn://svn.chromium.org/blink/trunk@176970 bbb929c8-8fbe-4397-9dbb-9b2b20218538