Adding wtf.trace.snapshotAll which can be used to asynchronously grab
snapshots from all contexts.
This allows for workers to have their snapshots taken and shown in the
UI. Unfortunately without performance.now this is a little useless right
now so I'm disabling it by default.

Work on #96.
Merge pull request #216 from google/workersnapshots

Adding wtf.trace.snapshotAll which can be used to asynchronously grab snapshots from all contexts.