Allow WebRTC to require Mac 10.7 deployments in GN builds.

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 8767aa4108b85a10780e1795aad7e88df0a1104d..23e2d83af94c59ca42bcc698f0e037c2a0dcd078 100644
--- a/build/config/mac/mac_sdk.gni
+++ b/build/config/mac/mac_sdk.gni
@@ -4,12 +4,17 @@
 
 import("//build/config/chrome_build.gni")
 
+# See https://bugs.chromium.org/p/webrtc/issues/detail?id=5453.
+# We can drop the rtc_require_mac_10_7_deployment flag when Chromium
+# also requires a 10.7 deployment target.
+import("//build_overrides/build.gni")
+
 declare_args() {
   # Minimum supported version of the Mac SDK.
   mac_sdk_min = "10.10"
 
   # Minimum supported version of OSX.
-  mac_deployment_target = "10.6"
+  mac_deployment_target = mac_deployment_target_build_override
 
   # 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