Clear bitmap from canvas after done drawing to it.

Change-Id: I0ebc8510021a2dd0b147ced7890d939ec47809f6
diff --git a/src/com/android/browser/BookmarkUtils.java b/src/com/android/browser/BookmarkUtils.java
index 2e96613..ca0cfbc 100644
--- a/src/com/android/browser/BookmarkUtils.java
+++ b/src/com/android/browser/BookmarkUtils.java
@@ -104,6 +104,7 @@
                 drawFaviconToCanvas(context, favicon, canvas, iconBounds, type);
             }
         }
+        canvas.setBitmap(null);
         return bm;
     }