Expose information on why a zone started or Scope.apply() happened

[jbdeboer]

Currently, apps have no insight into what is triggering Scope.apply() calls. We should expose that information, possibly in a separate debug mode.

e.g. what HTTP response triggered a new Zone, what Timer?

[jbdeboer]

We should group ScopeStats by Zone.

e.g.

Zone 1
    digest (15ms fields, 30ms evals, 45ms reaction)
    digest (15ms fields, 30ms evals, 5ms reaction)
    flush (100ms)
    asyncTasks (200ms)
    digest (15ms fields, 30ms evals, 0ms reaction)
    flush (0ms)

Zone 2
  digest (15ms fields, 30ms evals, 45ms reaction)
  flush (100ms)

...

Then, we should track why each zone was started e.g.

Zone 1: on-click handler
Zone 2: http response for songs.json