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After https://chromium-review.googlesource.com/c/chromium/src/+/1068514,
no code will try to find the specific ServiceWorkerDispatcherHost for a
given renderer process, so this CL removes the mapping management of
ServiceWorkerDispatcherHosts inside ServiceWorkerContextCore, in the
meantime simplifies ServiceWorkerDispatcherHost a little.

BUG=845341

Change-Id: Ie7747846570f64a7a4eda960878bbc0c97042952
Reviewed-on: https://chromium-review.googlesource.com/1075849Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#563090}