GitLab

Although, exceptions are not used in chromium,
some standard libraries depend on noexcept markers.
Because Optional is an alternative of standard library,
it is nice to have such markers.

BUG=784732
TEST=Ran trybot.

Change-Id: I958bc3f18e1d898d65cc106748795c6a27462650
Reviewed-on: https://chromium-review.googlesource.com/857358Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539012}