Fix CC flags in GN Windows build.

It was applying a GCC command line flag on Windows

R=brettw@chromium.org

Review URL: https://webrtc-codereview.appspot.com/46989004

Cr-Commit-Position: refs/heads/master@{#9049}
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index 46bacd8..6d05504 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -21,9 +21,11 @@
     "LOGGING=1",
   ]
 
-  # TODO(henrike): issue 3307, make rtc_base build without disabling
-  # these flags.
-  cflags_cc = [ "-Wno-non-virtual-dtor" ]
+  if (is_posix) {
+    # TODO(henrike): issue 3307, make rtc_base build without disabling
+    # these flags.
+    cflags_cc = [ "-Wno-non-virtual-dtor" ]
+  }
 }
 
 config("rtc_base_chromium_config") {