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This CL extends the manifest scope behavior so a server can override
the default scope.  The default scope for a new AppCache instance
is the manifest URL with an empty path.  An upcoming change will
update the default to be the manifest URL without the manifest
filename.

If a server wants to support the behavior that a manifest can specify
resources outside of that scope, the server can pass a response header
in their 200 or 304 response named X-AppCache-Allowed.  The value of
this header must be a URL path and have a last character equal to a
forward slash.

Bug: 999001
Change-Id: I0277c5ab0645346c4a7c6d5eec5ea67c9cbeeda2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935035
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721895}