Pack BitmapTextBlob a bit tighter

BUG=skia:

Review URL: https://codereview.chromium.org/1092973004
diff --git a/src/gpu/GrBatchAtlas.h b/src/gpu/GrBatchAtlas.h
index 889d260..77a49ce 100644
--- a/src/gpu/GrBatchAtlas.h
+++ b/src/gpu/GrBatchAtlas.h
@@ -102,8 +102,8 @@
 
         static const int kMinItems = 4;
         static const int kMaxPlots = 32;
-        uint32_t fPlotAlreadyUpdated;
         SkAutoSTMalloc<kMinItems, int> fPlotsToUpdate;
+        uint32_t fPlotAlreadyUpdated;
         int fCount;
         int fAllocated;