Implement converting constructors from Optional<U>.

BUG=784732
TEST=Ran trybot.

Change-Id: Icbb9c5596b045ca0c684cdad0343a2605d00dbb1
Reviewed-on: https://chromium-review.googlesource.com/856296
Reviewed-by: danakj <[email protected]>
Commit-Queue: Hidehiko Abe <[email protected]>
Cr-Commit-Position: refs/heads/master@{#533185}
4 files changed