ScopedVector - fix const semantics.

const std::vector<T*> returns a const reference to a T* for
operator[]. const ScopedVector<T> instead returns a const T*, which
means the two have slightly differing semantics.

Fix ScopedVector to behave like std::vector instead - the reference
to the pointer is const, but the pointer is not const.

TBR=willchan@chromium.org
R=brettw@chromium.org

Review URL: https://chromiumcodereview.appspot.com/18131002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@209537 0039d316-1c4b-4281-b951-d872f2087c98

base/memory/scoped_vector.h

@@ -41,8 +41,8 @@ class ScopedVector {
     return *this;
   }
 
-  T*& operator[](size_t index) { return v_[index]; }
-  const T* operator[](size_t index) const { return v_[index]; }
+  reference operator[](size_t index) { return v_[index]; }
+  const_reference operator[](size_t index) const { return v_[index]; }
 
   bool empty() const { return v_.empty(); }
   size_t size() const { return v_.size(); }