Retract GrRenderTarget from GrTestTarget

Split out of: https://codereview.chromium.org/1988923002/ (Declassify GrClipMaskManager and Remove GrRenderTarget and GrDrawTarget from GrPipelineBuilder)

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Review-Url: https://codereview.chromium.org/2004433002
diff --git a/include/gpu/GrDrawContext.h b/include/gpu/GrDrawContext.h
index 0e7211d..3778b72 100644
--- a/include/gpu/GrDrawContext.h
+++ b/include/gpu/GrDrawContext.h
@@ -296,6 +296,7 @@
     friend class GrStencilAndCoverTextContext; // for access to drawBatch
     friend class GrDrawingManager; // for ctor
     friend class GrDrawContextPriv;
+    friend class GrTestTarget;  // for access to getDrawTarget
 
     bool drawFilledDRRect(const GrClip& clip,
                           const GrPaint& paint,