Set GeometryBufferMapThreshold defaults

include/gpu/GrContextOptions.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrContextOptions_DEFINED
 #define GrContextOptions_DEFINED
 
 #include "SkTypes.h"
 
 struct GrContextOptions {
     GrContextOptions()
         : fDrawPathToCompressedTexture(false)
         , fSuppressPrints(false)
         , fMaxTextureSizeOverride(SK_MaxS32)
         , fSuppressDualSourceBlending(false)
-        , fGeometryBufferMapThreshold(1 << 15) {}
+        , fGeometryBufferMapThreshold(-1) {}
 
     // EXPERIMENTAL
     // May be removed in the future, or may become standard depending
     // on the outcomes of a variety of internal tests.
     bool fDrawPathToCompressedTexture;
 
     // Suppress prints for the GrContext.
     bool fSuppressPrints;
 
     /** Overrides: These options override feature detection using backend API queries. These
         overrides can only reduce the feature set or limits, never increase them beyond the
         detected values. */
 
     int  fMaxTextureSizeOverride;
     bool fSuppressDualSourceBlending;
 
-    /** fGeometryBufferMapThreshold gives a threshold (in bytes) for when Gr should
-        map a GrGeometryBuffer to update its contents. It will use map() if the
-        size of the updated region is greater than the threshold. Otherwise it will
-        use updateData(). */
-    size_t  fGeometryBufferMapThreshold;
+    /** the threshold in bytes above which we will use a buffer mapping API to map vertex and index
+        buffers to CPU memory in order to update them.  A value of -1 means the GrContext should
+        deduce the optimal value for this platform. */
+    int  fGeometryBufferMapThreshold;
 };
 
 #endif