Add fence support for TransferBuffers

src/gpu/gl/GrGLTestInterface.h

Index: src/gpu/gl/GrGLTestInterface.h
diff --git a/src/gpu/gl/GrGLTestInterface.h b/src/gpu/gl/GrGLTestInterface.h
index fc837bfec4902b5f5fbc7ba26fe60c2073d5bf18..ef00df397359346b561c58a914034d6fd973be37 100644
--- a/src/gpu/gl/GrGLTestInterface.h
+++ b/src/gpu/gl/GrGLTestInterface.h
@@ -317,6 +317,9 @@ public:
     virtual GrGLvoid* mapNamedBufferRange(GrGLuint buffer, GrGLintptr offset, GrGLsizeiptr length, GrGLbitfield access) { return nullptr; }
     virtual GrGLvoid flushMappedNamedBufferRange(GrGLuint buffer, GrGLintptr offset, GrGLsizeiptr length) {}
     virtual GrGLvoid textureBuffer(GrGLuint texture, GrGLenum target, GrGLenum internalformat, GrGLuint buffer) {}
+    virtual GrGLsync fenceSync(GrGLenum condition, GrGLbitfield flags) { return nullptr;  }
+    virtual GrGLenum clientWaitSync(GrGLsync sync, GrGLbitfield flags, GrGLuint64 timeout) { return GR_GL_WAIT_FAILED;  }
+    virtual GrGLvoid deleteSync(GrGLsync sync) {}
     virtual GrGLvoid debugMessageControl(GrGLenum source, GrGLenum type, GrGLenum severity, GrGLsizei count, const GrGLuint* ids, GrGLboolean enabled) {}
     virtual GrGLvoid debugMessageInsert(GrGLenum source, GrGLenum type, GrGLuint id, GrGLenum severity, GrGLsizei length,  const GrGLchar* buf) {}
     virtual GrGLvoid debugMessageCallback(GRGLDEBUGPROC callback, const GrGLvoid* userParam) {}