This change enables tcmalloc on Win64

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 2189 matching lines @@
             'AdditionalLibraryDirectories':
               ['<(windows_sdk_path)/Lib/win8/um/x64'],
           },
           'VCLibrarianTool': {
             'AdditionalLibraryDirectories!':
               ['<(windows_sdk_path)/Lib/win8/um/x86'],
             'AdditionalLibraryDirectories':
               ['<(windows_sdk_path)/Lib/win8/um/x64'],
           },
         },
-        'defines': [
-          # Not sure if tcmalloc works on 64-bit Windows.
-          'NO_TCMALLOC',
-        ],
       },
       'Debug_Base': {
         'abstract': 1,
         'defines': [
           'DYNAMIC_ANNOTATIONS_ENABLED=1',
           'WTF_USE_DYNAMIC_ANNOTATIONS=1',
         ],
         'xcode_settings': {
           'COPY_PHASE_STRIP': 'NO',
           'GCC_OPTIMIZATION_LEVEL': '<(mac_debug_optimization)',
@@ skipping 1830 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }