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This CL supports AppCache with PlzDedicatedWorker. Dedicated workers use their
ancestor frame's AppCacheHost, while shared workers have their own AppCacheHost.

The most tricky part of this CL is that dedicated workers are handled as
subresources of the nearest ancestor frame's AppCacheHost, but they also behave
as independent execution contexts. That is, dedicated workers have their own
subresource loaders. See comments in DedicatedWorkerHost and
AppCacheRequestHandler for details.

Change-Id: I0f24c57e94bf6fa71b8f15a22214ed91a96fbb17
Bug: 945673
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768485
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692392}