Set GeometryBufferMapThreshold defaults

TBR=bsalomon@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/1161543003
diff --git a/include/gpu/GrCaps.h b/include/gpu/GrCaps.h
index 38cf901..f219cb2 100644
--- a/include/gpu/GrCaps.h
+++ b/include/gpu/GrCaps.h
@@ -203,6 +203,7 @@
     }
 
     size_t geometryBufferMapThreshold() const {
+        SkASSERT(fGeometryBufferMapThreshold >= 0);
         return fGeometryBufferMapThreshold;
     }
 
@@ -229,6 +230,7 @@
 
     BlendEquationSupport fBlendEquationSupport;
     uint32_t fMapBufferFlags;
+    int fGeometryBufferMapThreshold;
 
     int fMaxRenderTargetSize;
     int fMaxTextureSize;
@@ -242,7 +244,6 @@
 private:
     bool fSupressPrints : 1;
     bool fDrawPathMasksToCompressedTextureSupport : 1;
-    size_t fGeometryBufferMapThreshold;
 
     typedef SkRefCnt INHERITED;
 };