mac, GN: Pass 10.6 as deployment target.

build/config/mac/mac_sdk.gni

Index: build/config/mac/mac_sdk.gni
diff --git a/build/config/mac/mac_sdk.gni b/build/config/mac/mac_sdk.gni
index 3f551087018eb9edbbe7547d790c00f71b481af6..20008d47975b7cdbf5be53fd19020d469d37a3b5 100644
--- a/build/config/mac/mac_sdk.gni
+++ b/build/config/mac/mac_sdk.gni
@@ -8,7 +8,10 @@ declare_args() {
   # Minimum supported version of the Mac SDK.
   mac_sdk_min = "10.10"
 
-  # Path to a specific version of the Mac SDKJ, not including a backslash at
+  # Minimum supported version of OSX.
+  mac_deployment_target = "10.6"
+
+  # Path to a specific version of the Mac SDK, not including a backslash at
   # the end. If empty, the path to the lowest version greater than or equal to
   # mac_sdk_min is used.
   mac_sdk_path = ""