Mark drawbitmaprect's large bitmaps as immutable.

This reduces the peak memory usage of out/Debug/dm --skr --match drawbitmaprect from ~1.6G to ~200M.

I'm not sure how the numbers got so huge to begin with from a 2K x 2K 8888 bitmap (16MB).

BUG=skia:2378
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/265813007

git-svn-id: http://skia.googlecode.com/svn/trunk@14529 2bbb7eff-a529-9590-31e7-b0007b416f81

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;