[ServiceWorker] Remove the legacy IPC messages file

The legacy IPC struct traits of ServiceWorkerFetchRequest is no longer
used anywhere, so we can just remove it then remove the whole file
service_worker_messages.h.

BUG=789854

Change-Id: I4d4e8ab30d1fca562d6b738aad394d1eb73935e4
Reviewed-on: https://chromium-review.googlesource.com/1187901
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#585726}
21 files changed