Fix minor cppcheck-found issues

https://codereview.chromium.org/13146005/



git-svn-id: http://skia.googlecode.com/svn/trunk@8440 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/utils/SkBitSet.h b/src/utils/SkBitSet.h
index 484fc2a..e113fd7 100755
--- a/src/utils/SkBitSet.h
+++ b/src/utils/SkBitSet.h
@@ -20,7 +20,7 @@
     explicit SkBitSet(int numberOfBits);
     explicit SkBitSet(const SkBitSet& source);
 
-    const SkBitSet& operator=(const SkBitSet& rhs);
+    SkBitSet& operator=(const SkBitSet& rhs);
     bool operator==(const SkBitSet& rhs);
     bool operator!=(const SkBitSet& rhs);