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Mojo's PlatformChannel implemented its own mechanism to allocate PA_HND
identifiers, and used the PA_USER0 namespace. Separately we had some
callers expecting to use statically-defined PA_USER0 identifiers to pass
application-specific handles, creating potential for clashes.

For features like base::FieldTrialList we also need a mechanism to pass
a handle to a child process at startup, for which we'd ideally have a
well-known path location in the FDIO namespace supplied to the child.
Since FDIO namespaces do not yet support non-file/directory handles, we
need a safe way to allocate an id that the FieldTrial implementation can
pass via the command-line, similarly to Mojo.

We add an AddHandleToTransfer() helper that appends a handle to a
handles-to-transfer vector with a unique id, and returns the id, and add
comments to clarify that PA_USER1 is used for these ids, leaving
PA_USER0 (and others) free for application use.

Bug: 909851
Change-Id: If9b8ffaf98155f72cc7a52e5ef0c51ef19048d66
Reviewed-on: https://chromium-review.googlesource.com/c/1354398Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612899}