Don't pass /safeseh to the linker when using clang on Windows

It does not generate SEH safe code at the moment.

R=scottmg@chromium.org
BUG=82385

Review URL: https://codereview.chromium.org/21910008

git-svn-id: http://src.chromium.org/svn/trunk/src/build@215743 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -4538,6 +4538,9 @@
                   ['asan==0', {
                     'AdditionalOptions': ['/largeaddressaware'],
                   }],
+                  ['clang==1', {
+                    'AdditionalOptions!': ['/safeseh'],
+                  }],
                 ],
               },
             },