add context override of GeometryBufferMapThreshold

BUG=skia:

Review URL: https://codereview.chromium.org/1159713006
diff --git a/src/gpu/GrCaps.cpp b/src/gpu/GrCaps.cpp
index f078fad..1f277b7 100644
--- a/src/gpu/GrCaps.cpp
+++ b/src/gpu/GrCaps.cpp
@@ -105,6 +105,7 @@
 
     fSupressPrints = options.fSuppressPrints;
     fDrawPathMasksToCompressedTextureSupport = options.fDrawPathToCompressedTexture;
+    fGeometryBufferMapThreshold = options.fGeometryBufferMapThreshold;
 }
 
 void GrCaps::applyOptionsOverrides(const GrContextOptions& options) {