Add struct index-lookup test.

Somewhat surprisingly, this test is ES2 compliant.

We didn't have any existing tests (AFAICS) which exercised nested
lookups having more than one index expression.

Change-Id: I869edc048744fad3a2a5587c8c9d70fc477c86ae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/648458
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
9 files changed