DevTools: make profiling lock global rather than per Target

We should pause any actions that could affect performance if any of the targets is being profiled, not only current target.

Compared to r180336 profiling lock is now initialized in Locks.js

BUG=403684
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/472263002

git-svn-id: svn://svn.chromium.org/blink/trunk@180351 bbb929c8-8fbe-4397-9dbb-9b2b20218538