Fix Dawn+Clang+GN build

For now [1] Dawn defaults spirv validation on all the time,
which caused an issue with some of their other CMake files.

[1] https://dawn-review.googlesource.com/c/dawn/+/309535

Bug: 502536510
Change-Id: I1fdcc712f0095e8d054c5f2c1c5e7fdb2f2ac814
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1239499
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>