Start using GrGpuCommandBuffer in GrDrawTarget.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2052263003

Review-Url: https://codereview.chromium.org/2078483002
diff --git a/src/gpu/GrBatchFlushState.cpp b/src/gpu/GrBatchFlushState.cpp
index c4ba264..4e51ae4 100644
--- a/src/gpu/GrBatchFlushState.cpp
+++ b/src/gpu/GrBatchFlushState.cpp
@@ -13,6 +13,7 @@
 GrBatchFlushState::GrBatchFlushState(GrGpu* gpu, GrResourceProvider* resourceProvider)
     : fGpu(gpu)
     , fResourceProvider(resourceProvider)
+    , fCommandBuffer(nullptr)
     , fVertexPool(gpu)
     , fIndexPool(gpu)
     , fLastIssuedToken(GrBatchDrawToken::AlreadyFlushedToken())