clang/win: It's ok to pass /GS to clang-cl now.

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 5684 matching lines @@
           # TODO(brucedawson): http://crbug.com/593448 - C4595 is an 'illegal
           # inline operator new' warning that is new in VS 2015 Update 2.
           # This is equivalent to clang's no-inline-new-delete warning.
           # See http://bugs.icu-project.org/trac/ticket/11122
           4595,
         ],
         'msvs_settings': {
           'VCCLCompilerTool': {
             'AdditionalOptions': ['/MP'],
             'MinimalRebuild': 'false',
+            'BufferSecurityCheck': 'true',
             'EnableFunctionLevelLinking': 'true',
             'RuntimeTypeInfo': 'false',
             'WarningLevel': '4',
             'WarnAsError': 'true',
             'DebugInformationFormat': '3',
             # ExceptionHandling must match _HAS_EXCEPTIONS above.
             'ExceptionHandling': '0',
           },
           'VCLibrarianTool': {
             'AdditionalOptions': ['/ignore:4221'],
@@ skipping 52 matching lines @@
             }],
             ['_type=="executable" and ">(win_exe_compatibility_manifest)"!=""', {
               'VCManifestTool': {
                 'AdditionalManifestFiles': [
                   '>(win_exe_compatibility_manifest)',
                 ],
               },
             }],
           ],
           'conditions': [
-            ['clang==0', {
-              'VCCLCompilerTool': {
-                 # TODO(thakis): Enable this with clang too,
-                 # https://crbug.com/598767
-                 'BufferSecurityCheck': 'true',
-              },
-            }],
-
             # Building with Clang on Windows is a work in progress and very
             # experimental. See crbug.com/82385.
             # Keep this in sync with the similar blocks in build/config/compiler/BUILD.gn
             ['clang==1', {
               'VCCLCompilerTool': {
                 'AdditionalOptions': [
                   # Don't warn about unused function parameters.
                   # (This is also used on other platforms.)
                   '-Wno-unused-parameter',
                   # Don't warn about the "struct foo f = {0};" initialization
@@ skipping 527 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',
   },
 }