Add transfer buffer support.

BUG=skia:4604

Committed: https://skia.googlesource.com/skia/+/fa498fe12239988578465d0dee69f0d5645bb361

Review URL: https://codereview.chromium.org/1490473003
diff --git a/src/gpu/GrTest.cpp b/src/gpu/GrTest.cpp
index b1c1354..a3823dd 100644
--- a/src/gpu/GrTest.cpp
+++ b/src/gpu/GrTest.cpp
@@ -311,6 +311,8 @@
 
     GrIndexBuffer* onCreateIndexBuffer(size_t size, bool dynamic) override { return nullptr; }
 
+    GrTransferBuffer* onCreateTransferBuffer(size_t, TransferType) override { return nullptr; }
+
     void onClear(GrRenderTarget*, const SkIRect& rect, GrColor color) override {}
 
     void onClearStencilClip(GrRenderTarget*, const SkIRect& rect, bool insideClip) override {}