[ganesh][dawn] Fix -Wreturn-std-move-in-c++11 error

Change-Id: Ic47605588203df2a765b241c5c366973741b8867
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/536100
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>

src/gpu/ganesh/dawn/GrDawnGpu.cpp

@@ -184,7 +184,7 @@ sk_sp<GrGpuBuffer> GrDawnGpu::onCreateBuffer(size_t size, GrGpuBufferType type,
     if (data && buffer) {
         buffer->updateData(data, size);
     }
-    return buffer;
+    return std::move(buffer);
 }
 
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