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This also adds a move constructor (since there's no obvious reason not
to, and Optional has them) and copy and move assignment (since
constructors and assignments should be defined together per style guide).

This also makes the comparison operators non-members since this is
preferred when feasible (e.g. makes implicit conversion for comparisons
apply equally to both sides).

Bug: none
Change-Id: I5b593f078063b8f9502dae232ed28e01ae41cd5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026688
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736272}