Implement the deference operator for scoped_refptr.

scoped_refptr previously got this for free since it implemented operator
T*, but the conversion operator is being removed. This prevents awkward
looking code like *my_favorite_pointer.get().

BUG=110610

Review URL: https://codereview.chromium.org/505943002

Cr-Commit-Position: refs/heads/master@{#291869}

base/memory/ref_counted.h

@@ -295,6 +295,11 @@ class scoped_refptr {
   // and comparison operations.
   operator T*() const { return ptr_; }
 
+  T& operator*() const {
+    assert(ptr_ != NULL);
+    return *ptr_;
+  }
+
   T* operator->() const {
     assert(ptr_ != NULL);
     return ptr_;