Clear bitmap from canvas after done drawing to it.

Change-Id: I0ebc8510021a2dd0b147ced7890d939ec47809f6
diff --git a/src/com/android/browser/Controller.java b/src/com/android/browser/Controller.java
index 0d22af0..142375d 100644
--- a/src/com/android/browser/Controller.java
+++ b/src/com/android/browser/Controller.java
@@ -1967,6 +1967,7 @@
         thumbnail.draw(canvas);
         Bitmap ret = Bitmap.createScaledBitmap(bm, width / filter_scale,
                 height / filter_scale, true);
+        canvas.setBitmap(null);
         bm.recycle();
         return ret;
     }