Add nonstd::unique_ptr move assigment operator.

third_party/base/nonstd_unique_ptr.h

Index: third_party/base/nonstd_unique_ptr.h
diff --git a/third_party/base/nonstd_unique_ptr.h b/third_party/base/nonstd_unique_ptr.h
index 1d1c43f42fb4ac706cb1ce01795b03f986e6c58a..d666e1eeb2586d093a496cd29eaf51f2a37d7c9a 100644
--- a/third_party/base/nonstd_unique_ptr.h
+++ b/third_party/base/nonstd_unique_ptr.h
@@ -176,6 +176,13 @@ class unique_ptr : public unique_ptr_base<C> {
     }
   }
 
+  // Move assignment.
+  unique_ptr<C>& operator=(unique_ptr<C>&& that) {
+    if (that.ptr_ != ptr_)
+      reset(that.release());
+    return *this;
+  }
+
 private:
   // Forbid comparison of unique_ptr types.  If C2 != C, it totally doesn't
   // make sense, and if C2 == C, it still doesn't make sense because you should
@@ -222,6 +229,13 @@ class unique_ptr<C[]> : public unique_ptr_base<C> {
     }
   }
 
+  // Move assignment.
+  unique_ptr<C>& operator=(unique_ptr<C>&& that) {
+    if (that.ptr_ != ptr_)
+      reset(that.release());
+    return *this;
+  }
+
   // Support indexing since it is holding array.
   C& operator[] (size_t i) { return ptr_[i]; }