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>

src/gpu/mtl/GrMtlGpu.mm

@@ -496,7 +496,7 @@ bool GrMtlGpu::clearTexture(GrMtlTexture* tex, size_t bpp, uint32_t levelMask) {
     GrStagingBufferManager::Slice slice = fStagingBufferManager.allocateStagingBufferSlice(
             combinedBufferSize, alignment);
     if (!slice.fBuffer) {
-        return nullptr;
+        return false;
     }
     GrMtlBuffer* mtlBuffer = static_cast<GrMtlBuffer*>(slice.fBuffer);
     id<MTLBuffer> transferBuffer = mtlBuffer->mtlBuffer();