[ServiceWorker] All provider hosts are owned by context core from birth

A provider host on creation is always added into
ServiceWorkerContextCore and owned there, later it'll destroy either
when ServiceWorkerContextCore destroys or the Mojo connection it's bound
breaks.
This CL refactors code to strengthen the logic above.

BUG=931092

Change-Id: I8feaebec0e1c539ad5e24bde3af7518e2f81aa17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1522559
Commit-Queue: Leon Han <leon.han@intel.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641037}
2 files changed