Found multiple issues (added TODOs for them):
- isPaused() check is global, so one can resume from another session/context group
  without receiving 'paused' notification;
- setBreakpointsActive flag is global affecting all sessions and context groups;
- max async call stack depth is global, and should be per context group.

BUG=chromium:590878

Review-Url: https://codereview.chromium.org/2921373002
Cr-Commit-Position: refs/heads/master@{#45742}