Switch GrMeshDrawOp::Target to be the stand alone GrMeshDrawTarget class

The Tessellator classes will survive in the NGA and they use
GrMeshDrawOp::Target - but GrMeshDrawOp will not survive.

This is a prelude to making all the remaining GrOp-derived classes OGA-only.

Bug: skia:11837
Change-Id: I62dc92e5f42d672342113f12dcedf3435fab993f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/419198
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
diff --git a/src/gpu/GrThreadSafeCache.h b/src/gpu/GrThreadSafeCache.h
index 8b08ff5..b44e706 100644
--- a/src/gpu/GrThreadSafeCache.h
+++ b/src/gpu/GrThreadSafeCache.h
@@ -13,10 +13,9 @@
 #include "src/core/SkArenaAlloc.h"
 #include "src/core/SkTDynamicHash.h"
 #include "src/core/SkTInternalLList.h"
+#include "src/gpu/GrGpuBuffer.h"
 #include "src/gpu/GrSurfaceProxyView.h"
 
-class GrGpuBuffer;
-
 // Ganesh creates a lot of utility textures (e.g., blurred-rrect masks) that need to be shared
 // between the direct context and all the DDL recording contexts. This thread-safe cache
 // allows this sharing.