service worker: Remove unused PROVIDER_FOR_SANDBOXED_IFRAME

Clean-up only. This was added in https://codereview.chromium.org/1191293002/
then became unused in https://codereview.chromium.org/1399363004. Originally it
signaled to the ServiceWorkerNetworkProvider ctor that the provider id should
be set to invalid; now the default ctor is used accomplish that.

BUG=

Review-Url: https://codereview.chromium.org/2023733002
Cr-Commit-Position: refs/heads/master@{#396685}
4 files changed