Fix SwitchWithFallthrough test on iOS.

It looks like returning from inside a switch on iOS gives wrong results
in GLSL.

Change-Id: I9d6d8971a7a54600268e27443815444fca6f3c61
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/450994
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed