Fix for crashing bots

TBR=bsalomon@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/1159183006

src/gpu/GrTest.cpp

@@ -239,6 +239,7 @@ GrContext* GrContext::CreateMockContext() {
 
 void GrContext::initMockContext() {
     GrContextOptions options;
+    options.fGeometryBufferMapThreshold = 0;
     SkASSERT(NULL == fGpu);
     fGpu = SkNEW_ARGS(MockGpu, (this, options));
     SkASSERT(fGpu);