Correct failure return for GrMtlGpu::clearTexture
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 <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Reviewed-by: Greg Daniel <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>

google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia