remoting: Suppress a clang warning that fires in atl headers.

remoting/remoting_host_win.gypi

Index: remoting/remoting_host_win.gypi
diff --git a/remoting/remoting_host_win.gypi b/remoting/remoting_host_win.gypi
index 91e13e5a28197425a39b8771c1d2e8184470fbde..a3f0777262406aa66924240f3f673acb070e6d97 100644
--- a/remoting/remoting_host_win.gypi
+++ b/remoting/remoting_host_win.gypi
@@ -250,6 +250,16 @@
                 '/EXPORT:DllUnregisterServer=PsDllUnregisterServer,PRIVATE',
               ],
             },
+            'conditions': [
+              ['clang==1', {
+                # atlbase.h contains a global "using namespace WTL;".
+                # TODO: Remove once remoting/host/verify_config_window_win.h no
+                # longer depends on atlbase.h, http://crbug.com/5027
+                'VCCLCompilerTool': {
+                  'AdditionalOptions': ['-Wno-header-hygiene'],
+                },
+              }],
+            ],
           },
         },  # end of target 'remoting_core'
         {