Add explicit clear of stencil buffer before opLists that use them

Change-Id: I9e2468e1331c6593dbc6da3ad510f08d1c589e8d
Reviewed-on: https://skia-review.googlesource.com/32041
Commit-Queue: Stan Iliev <stani@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/mock/GrMockGpuCommandBuffer.h b/src/gpu/mock/GrMockGpuCommandBuffer.h
index 0213cb9..d2f6baf 100644
--- a/src/gpu/mock/GrMockGpuCommandBuffer.h
+++ b/src/gpu/mock/GrMockGpuCommandBuffer.h
@@ -20,6 +20,7 @@
                       GrRenderTargetProxy*) override {}
     void discard(GrRenderTargetProxy*) override {}
     void insertEventMarker(GrRenderTargetProxy*, const char*) override {}
+    void begin() override {}
     void end() override {}
 
     int numDraws() const { return fNumDraws; }