[dawn] Disable SPIR-V validation

SPIR-V validation is soon going to be enabled by default in Dawn on
Android. Since this may add some overhead to pipeline creation, this
toggle can be used to disable that validation. This is safe for
Graphite since the input shaders are trusted.

Fixed: 473526493
Change-Id: If49592c0d41298a9fcfd407de1dffff8ea48b7e3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1142576
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: James Price <jrprice@google.com>