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On PlzWorker, AppCacheHost is created before starting worker script loading from
a renderer process. This CL teaches the precreated AppCacheHost's ID from the
browser process to the renderer process in order to associate worker script
loading with the host.

This fixes DCHECK failures on appcache-worker.https.html caused by host
mismatch, the test still times out though.

DesignDoc:
https://docs.google.com/document/d/1Jtn33bvqkqWxq6K7HIA4uU6HLWPTmOD7vFviacfTmhM/edit?usp=sharing

Bug: 715632
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I16c17447bbb0f4b154c5a7e9393afdea2d0e29d6
Reviewed-on: https://chromium-review.googlesource.com/1150941
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578547}