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Because the input type to OnceCallback::Then() is a templated type as
OnceCallback<U>, a RepeatingCallback does not bind and implicitly
convert to a OnceCallback. This is because templated pattern matching
does not try conversions, so it must match exactly.

Add an overload of OnceCallback::Then() to receive a RepeatingCallback
and cast it explicitly to a OnceCallback, forwarding it on to the
original Then() method.

R=dcheng@chromium.org

Bug: 1140582
Change-Id: Ib571eb361922588a3f200e6aea5c045aad0f72d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510411
Commit-Queue: danakj <danakj@chromium.org>
Auto-Submit: danakj <danakj@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822850}