Mark drawbitmaprect's large bitmaps as immutable.

gm/drawbitmaprect.cpp

Index: gm/drawbitmaprect.cpp
diff --git a/gm/drawbitmaprect.cpp b/gm/drawbitmaprect.cpp
index bc7e0359ed349797a8565b9e62b26ec7100846ee..d9b1434e57c3e236c4dc80f22c673aaccdd0ac42 100644
--- a/gm/drawbitmaprect.cpp
+++ b/gm/drawbitmaprect.cpp
@@ -68,6 +68,8 @@ static void makebm(SkBitmap* bm, int w, int h) {
         rect.inset(wScalar / 8, hScalar / 8);
         mat.postScale(SK_Scalar1 / 4, SK_Scalar1 / 4);
     }
+    // Let backends know we won't change this, so they don't have to deep copy it defensively.
+    bm->setImmutable();
 }
 
 static const int gSize = 1024;