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This makes it so optional mojom values can only be read from their
DataView into a destination type that supports nullability when read
with the common Read[FieldName] methods. Attempting to violate this
constraint will result in a compile-time assertion failure at the site
of the Read[FieldName] call. Prior to this change, such instances were
non-obvious and could instead result in surprising runtime behavior.

Types that support nullability include anything wrapped with
base::Optional, or any type for which there exists an appropriate
{Struct,Union,Array,String}Traits definition with a SetToNull method.

Pre-existing violations of the new constraint are corrected here either
by making fields non-optional, deserializing to a base::Optional, or
doing something more specialized to fix traits logic.

Finally, this also removes the IsNull and SetToNull methods from
StringTraits<std::string> since: (a) optional string fields are
generated as base::Optional<std::string> and (b) it was not intentional
for null strings and empty strings to be treated as equivalent in
meaning.

Fixed: 1124639
Change-Id: I0569b4b8420b4b416bd889417d9723307a880454
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404478
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807146}