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Add a test that verifies that runtime.onStartup fires on Service
Worker based extension upon chrome start.

This CL also expands "runtime" feature to "extension_service_worker"
context type.

Bug: 891016
Change-Id: I4a11f6da2f5311d7901ad8bb05c348b9343fe701
Reviewed-on: https://chromium-review.googlesource.com/1255588
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595708}