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For running PendingTasks for a particular ContextId through
SWTQ::AddPendingTask(), call StartWorker* exactly once instead of calling it
each time for each AddPendingTask call. This has two advantages:
  - We don't call StartWorker* more than once when we don't need to.
  - Extension messaging subtly depends on running these tasks once. This is
    because how LazyBackgroundTaskQueue behaves in terms of multiple pending
    tasks and we need some sort of parity in SWTQ for future worker messaging
    implementation.

Detailed changes:
- ServiceWorkerTaskQueue: Remove the queue for registration
  (old |pending_tasks_|) and worker start (|waiting_did_start_worker_tasks_|)
  and merge them to a single queue (|pending_tasks_). Run these tasks
  once registration, service worker start (DidStartWorkerForScope) and
  service worker loadstop (DidStartServiceWorkerContext) completes.
- IPC: Add thread_id and SW scope to DidStartServiceWorkerContext IPC
  (loadstop), as PendingTasks that were pending after DidStartWorkerForScope
  but before the SW context has seen loadstop can use these thread_id and
  scope to build LazyContextTaskQueue::ContextInfo to run those tasks.
  Previously we used to remember these two pieces of information in
  WaitingDidStartWorkerTask. This also simplifies the queue in SWTQ.
- IPC: Add thread_id and SW scope to DidStopServiceWorkerContext IPC
  to clean up the task containers on worker shutdown. This will also
  be necessary for multiple worker support within extension in future.

Bug: 925927
Test: Service worker within extension internal change, no visible effect.
Change-Id: I06581f821550154ebca3ee6d9aa803953abf7d86
Reviewed-on: https://chromium-review.googlesource.com/c/1481580Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636239}