mojo: Don't leak handles when assigning to ScopedHandles.

mojo/public/cpp/system/core.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_SYSTEM_CORE_H_
 #define MOJO_PUBLIC_CPP_SYSTEM_CORE_H_
 
 #include <assert.h>
 #include <stddef.h>
 
@@ skipping 89 matching lines @@
   ~ScopedHandleBase() { CloseIfNecessary(); }
 
   template <class CompatibleHandleType>
   explicit ScopedHandleBase(ScopedHandleBase<CompatibleHandleType> other)
       : handle_(other.release()) {
   }
 
   // Move-only constructor and operator=.
   ScopedHandleBase(RValue other) : handle_(other.object->release()) {}
   ScopedHandleBase& operator=(RValue other) {
-    handle_ = other.object->release();
+    if (other.object != this) {
+      CloseIfNecessary();
+      handle_ = other.object->release();
+    }
     return *this;
   }
 
   const HandleType& get() const { return handle_; }
 
   template <typename PassedHandleType>
   static ScopedHandleBase<HandleType> From(
       ScopedHandleBase<PassedHandleType> other) {
     MOJO_COMPILE_ASSERT(
         sizeof(static_cast<PassedHandleType*>(static_cast<HandleType*>(0))),
@@ skipping 421 matching lines @@
       CreateSharedBuffer(&options, num_bytes, &handle);
   assert(result == MOJO_RESULT_OK);
 }
 
 inline SharedBuffer::~SharedBuffer() {
 }
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_SYSTEM_CORE_H_