Some simple optimizations for improving GrAtlasTextContext perf

BUG=skia:

Review URL: https://codereview.chromium.org/1093083003
diff --git a/src/gpu/GrBatchAtlas.h b/src/gpu/GrBatchAtlas.h
index 77a49ce..2b85da1 100644
--- a/src/gpu/GrBatchAtlas.h
+++ b/src/gpu/GrBatchAtlas.h
@@ -112,6 +112,11 @@
 
     void setLastUseTokenBulk(const BulkUseTokenUpdater& reffer, BatchToken);
 
+    static const int kGlyphMaxDim = 256;
+    static bool GlyphTooLargeForAtlas(int width, int height) {
+        return width > kGlyphMaxDim || height > kGlyphMaxDim;
+    }
+
 private:
     static int GetIndexFromID(AtlasID id) {
         return id & 0xffff;