Create stub GrAuditTrail class

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1572553002

Review URL: https://codereview.chromium.org/1572553002
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 39477e0..a49d141 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -17,6 +17,7 @@
 #include "SkMatrix.h"
 #include "SkPathEffect.h"
 #include "SkTypes.h"
+#include "../private/GrAuditTrail.h"
 #include "../private/GrSingleOwner.h"
 #include "../private/SkMutex.h"
 
@@ -358,6 +359,8 @@
     /** Get pointer to atlas texture for given mask format */
     GrTexture* getFontAtlasTexture(GrMaskFormat format);
 
+    SkString dumpAuditTrailToJson() const { return fAuditTrail.toJson(); }
+
 private:
     GrGpu*                          fGpu;
     const GrCaps*                   fCaps;
@@ -405,6 +408,8 @@
 
     SkAutoTDelete<GrDrawingManager> fDrawingManager;
 
+    GrAuditTrail                    fAuditTrail;
+
     // TODO: have the CMM use drawContexts and rm this friending
     friend class GrClipMaskManager; // the CMM is friended just so it can call 'drawingManager'
     friend class GrDrawingManager;  // for access to drawingManager for ProgramUnitTest