Fix GPU trace marker bug

Without this patch the GPU trace markers can be potentially unbalanced when batching occurs.

Review URL: https://codereview.chromium.org/981973002
diff --git a/src/gpu/GrTargetCommands.cpp b/src/gpu/GrTargetCommands.cpp
index d326226..30920f8 100644
--- a/src/gpu/GrTargetCommands.cpp
+++ b/src/gpu/GrTargetCommands.cpp
@@ -319,6 +319,10 @@
         if (Cmd::kDrawBatch_Cmd == iter->type()) {
             DrawBatch* db = reinterpret_cast<DrawBatch*>(iter.get());
             fBatchTarget.flushNext(db->fBatch->numberOfDraws());
+
+            if (iter->isTraced()) {
+                gpu->removeGpuTraceMarker(&newMarker);
+            }
             continue;
         }