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Instead of forcing ArgType<T>, just generically match a signature type.
This allows classes to choose to implement e.g. by reference or value,
at the cost of possibly-more-cryptic template errors when people don't
implement a setter.

We could probably give better errors than this with some helper structs
to restrict to e.g. single-arg member functions whose arg is convertible
from ArgType, but this seems good enough for now.

Bug: none
Change-Id: I079fd9dd5cd3cb51297855d2ccabde65148c0b6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363562
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801146}