net: disable ECDSA ciphersuites on platforms where we can't support it.

base/mac/mac_util.mm

Index: base/mac/mac_util.mm
diff --git a/base/mac/mac_util.mm b/base/mac/mac_util.mm
index a0aa72c88728f54e052683c48840638d5e64ba07..08c9a173359b7e5075e8cf6997264868c7a5d03b 100644
--- a/base/mac/mac_util.mm
+++ b/base/mac/mac_util.mm
@@ -573,6 +573,12 @@ bool IsOSSnowLeopard() {
 }
 #endif
 
+#if !defined(BASE_MAC_MAC_UTIL_H_INLINED_GE_10_7)
+bool IsOSSnowLeopardOrLater() {
+  return MacOSXMinorVersion() >= SNOW_LEOPARD_MINOR_VERSION;
+}
+#endif
+
 #if !defined(BASE_MAC_MAC_UTIL_H_INLINED_GT_10_7)
 bool IsOSLion() {
   return MacOSXMinorVersion() == LION_MINOR_VERSION;