ScopedPlatformHandle encapsulates ownership of one of various types
of platform-specific resource handles including POSIX file descriptors,
and Windows handles.

The purpose of this type is to have a consistent representation of such
resources for the sake of safe ownership semantics across various API
boundaries, relegating platform-specific details to either the code which
initially acquires the resource or the code which actually uses it.

BUG=None

Review-Url: https://codereview.chromium.org/2705743002
Cr-Commit-Position: refs/heads/master@{#453242}