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.cpp b/src/utils/SkBitSet.cpp
index 664c8cd..36ff6cb 100755
--- a/src/utils/SkBitSet.cpp
+++ b/src/utils/SkBitSet.cpp
@@ -23,7 +23,7 @@
     *this = source;
 }
 
-const SkBitSet& SkBitSet::operator=(const SkBitSet& rhs) {
+SkBitSet& SkBitSet::operator=(const SkBitSet& rhs) {
     if (this == &rhs) {
         return *this;
     }