[base] Further improve the logging facility.

src/checks.h

 // Copyright 2012 the V8 project 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 V8_CHECKS_H_
 #define V8_CHECKS_H_
 
 #include "include/v8.h"
 #include "src/base/logging.h"
 
 namespace v8 {
-
-class Value;
-template <class T> class Handle;
-
 namespace internal {
 
 #ifdef ENABLE_SLOW_DCHECKS
 #define SLOW_DCHECK(condition) \
   CHECK(!v8::internal::FLAG_enable_slow_asserts || (condition))
 extern bool FLAG_enable_slow_asserts;
 #else
 #define SLOW_DCHECK(condition) ((void) 0)
 const bool FLAG_enable_slow_asserts = false;
 #endif
 
 } }  // namespace v8::internal
 
 #define DCHECK_TAG_ALIGNED(address)             \
   DCHECK((reinterpret_cast<intptr_t>(address) & \
           ::v8::internal::kHeapObjectTagMask) == 0)
 
 #define DCHECK_SIZE_TAG_ALIGNED(size) \
   DCHECK((size & ::v8::internal::kHeapObjectTagMask) == 0)
 
 #endif  // V8_CHECKS_H_