Correct failure return for GrMtlGpu::clearTexture

GrMtlGpu::clearTexture returns bool. The Chromium compile complains
about returning nullptr instead.

Change-Id: I27d70fbe75edf7ba172ea8e63843cfaf7b93cac3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/423996
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
1 file changed