add context override of GeometryBufferMapThreshold

src/gpu/GrBufferAllocPool.h

Index: src/gpu/GrBufferAllocPool.h
diff --git a/src/gpu/GrBufferAllocPool.h b/src/gpu/GrBufferAllocPool.h
index df0a0ccd9c870facc61056278abf82fa78af8642..cd5d1971da5722dcc0597788150e038c7f1c1354 100644
--- a/src/gpu/GrBufferAllocPool.h
+++ b/src/gpu/GrBufferAllocPool.h
@@ -128,6 +128,7 @@ private:
     int                             fPreallocBufferStartIdx;
     SkAutoMalloc                    fCpuData;
     void*                           fBufferPtr;
+    size_t                          fGeometryBufferMapThreshold;
 };
 
 class GrVertexBuffer;