Support clang-cl build on Windows.

build/standalone.gypi

Index: build/standalone.gypi
diff --git a/build/standalone.gypi b/build/standalone.gypi
index 1e7c0c0dc03a4fbfd4f7f71333b707584c622500..dd3e8e4fe3245f4d7d1ea461df5aaac970af26fd 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -318,6 +318,43 @@
           }, {
             'WarnAsError': 'true',
           }],
+          ['clang==1', {
+            '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
+              # pattern.
+              '-Wno-missing-field-initializers',
+
+              # Many files use intrinsics without including this header.
+              # TODO(hans): Fix those files, or move this to sub-GYPs.
+              '/FIIntrin.h',
+
+              # TODO(hans): Make this list shorter eventually, http://crbug.com/504657
+              '-Qunused-arguments',  # http://crbug.com/504658
+              '-Wno-microsoft-enum-value',  # http://crbug.com/505296
+              '-Wno-unknown-pragmas',  # http://crbug.com/505314
+              '-Wno-microsoft-cast',  # http://crbug.com/550065
+
+              # TODO(ochang): Fix these properly eventually.
+              '-Wno-inconsistent-missing-override',
+            ],
+          }],
+          ['clang==1 and MSVS_VERSION == "2013"', {
+            'VCCLCompilerTool': {
+              'AdditionalOptions': [
+                '-fmsc-version=1800',
+              ],
+            },
+          }],
+          ['clang==1 and MSVS_VERSION == "2015"', {
+            'VCCLCompilerTool': {
+              'AdditionalOptions': [
+                '-fmsc-version=1900',
+              ],
+            },
+          }],
         ],
       },
       'VCLibrarianTool': {
@@ -454,6 +491,15 @@
         }],
       ],
     }],  # OS=="linux" or OS=="mac"
+    ['OS=="win"', {
+      'conditions': [
+        ['clang==1', {
+          'make_global_settings': [
+            ['CC', '<(clang_dir)/bin/clang-cl'],
+          ],
+        }],
+      ],
+    }],  # OS=="win"
     ["use_goma==1", {
       'make_global_settings': [
         ['CC_wrapper', '<(gomadir)/gomacc'],