add SkRect::joinNonEmptyArg for faster unioning

BUG=skia:

Review URL: https://codereview.chromium.org/619853005
diff --git a/src/gpu/GrBitmapTextContext.cpp b/src/gpu/GrBitmapTextContext.cpp
index 422a7e0..c9cdf2c 100755
--- a/src/gpu/GrBitmapTextContext.cpp
+++ b/src/gpu/GrBitmapTextContext.cpp
@@ -575,7 +575,7 @@
     r.fRight = SkFixedToFloat(vx + width);
     r.fBottom = SkFixedToFloat(vy + height);
 
-    fVertexBounds.growToInclude(r);
+    fVertexBounds.joinNonEmptyArg(r);
 
     size_t vertSize = useColorVerts ? (2 * sizeof(SkPoint) + sizeof(GrColor)) :
                                       (2 * sizeof(SkPoint));