Capturing struct members is vague In ES 3.00. But the ES 3.10
explicitly says that base-level members of struct are feasible for
transform feedback capture. This implementation fills the gap.

TEST=angle_end2end_tests:TrasnformFeedbackTest*
BUG=angleproject:2241
Change-Id: Ibdf3ae6c2b8b28952e2f7fef1363545cbccad389
Reviewed-on: https://chromium-review.googlesource.com/768613
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>