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This is a step toward to killing FileDescriptor.
This change lets FiileDescriptorSet have both Files (for owning fds)
and PlatformFiles (for non-owning fds). Doing this, we no longer
need FileDescriptor which provides |auto_close| flag.

BUG=415294
TEST=ipc_tests, ipc_mojo_unittests
R=agl@chromium.org, jam@hcromium.org, viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/583473002

Cr-Commit-Position: refs/heads/master@{#296498}