mark bitmap immutable in bench

So we don't incidentally measure the cost of the copy

Change-Id: I6f7e2119792423b0857764b3b557a9f838bb43fb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334423
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>

bench/BitmapBench.cpp

@@ -68,6 +68,9 @@ protected:
         this->onDrawIntoBitmap(bm);
 
         fBitmap = bm;
+        if (!fForceUpdate) {
+            fBitmap.setImmutable();
+        }
     }
 
     void onDraw(int loops, SkCanvas* canvas) override {