Make sure to set a uniform's flattenedOffsetInParentArrays
value only if it has a valid parent array index.
Serialize/Deserialize flattenedOffsetInParentArrays value.

Bug: angleproject:3216
Bug: angleproject:3217
Bug: angleproject:3665
Change-Id: I6e4d003b1dd74b330bcf153f8d3df92414f198dd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1704219
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>