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Change function signatures.  This is mostly using Optional to
better convey "function returns a value or failure", but also uses a
struct in one spot to change outparams to multiple return values.

This change exposed what I think was previously a bug, where
EnumMonitorForDisplayInfoCallback() would use
GetDefaultMonitorPhysicalPixelsPerInch() if it could not get an
appropriate pointer device, but would use an empty vector if
GetPixelsPerInchForPointerDevice() failed.  Using Optional for its
helpers makes it easy to use the former in both cases, which seems
more correct.

Bug: none
Change-Id: Ie9df3da798e1c5b448b57963929955193a2a255e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2109262
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752684}