The spec isn't very explicit about disallowing this, but conversions
from structs are not among the conversion constructors or specified
in any other way either.

BUG=angleproject:2036
TEST=angle_unittests

Change-Id: I23f2ceda1d1348cec0d3bba38a7a013275ff84eb
Reviewed-on: https://chromium-review.googlesource.com/514002
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>