mac, GN: Pass 10.6 as deployment target.

Previously, GN was using the sdk version as the deployment target,
which meant gn-built binaries couldn't run on OS X versions
older than 10.10 after updating the SDK to 10.10.
With this fix, mac GN matches the behavior of Mac GYP.

BUG=463170

Review URL: https://codereview.chromium.org/1285493004

Cr-Original-Commit-Position: refs/heads/master@{#342908}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e194adc1f7bdeeb500dcd357283d512651b6bf1b

config/mac/BUILD.gn

@@ -9,7 +9,7 @@ config("sdk") {
   common_flags = [
     "-isysroot",
     sysroot,
-    "-mmacosx-version-min=$mac_sdk_min",
+    "-mmacosx-version-min=$mac_deployment_target",
   ]
 
   cflags = common_flags

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 = ""