add SkRect::joinNonEmptyArg for faster unioning

src/gpu/GrDistanceFieldTextContext.cpp

Index: src/gpu/GrDistanceFieldTextContext.cpp
diff --git a/src/gpu/GrDistanceFieldTextContext.cpp b/src/gpu/GrDistanceFieldTextContext.cpp
index 07e9a2e21d7e2ed25025fec286ab284319c488d2..b565dd63f80476060fdd5c05684286a6b60f5470 100755
--- a/src/gpu/GrDistanceFieldTextContext.cpp
+++ b/src/gpu/GrDistanceFieldTextContext.cpp
@@ -411,7 +411,7 @@ HAS_ATLAS:
     r.fRight = sx + width;
     r.fBottom = sy + height;
 
-    fVertexBounds.growToInclude(r);
+    fVertexBounds.joinNonEmptyArg(r);
 
     size_t vertSize = fUseLCDText ? (2 * sizeof(SkPoint))
                                   : (2 * sizeof(SkPoint) + sizeof(GrColor));