[Mac/GN] Print a warning if the SDK version is 10.11 but Xcode is <7.3.

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 986bf7942a39342c7f8dbd3d88349b5f56fbe299..eb0fdda61492d095fc13f6586316e4b11162610f 100644
--- a/build/config/mac/mac_sdk.gni
+++ b/build/config/mac/mac_sdk.gni
@@ -51,3 +51,28 @@ mac_sdk_build = _mac_sdk_result.sdk_build
 xcode_version = _mac_sdk_result.xcode_version
 xcode_build = _mac_sdk_result.xcode_build
 machine_os_build = _mac_sdk_result.machine_os_build
+
+if (mac_sdk_version != mac_sdk_min_build_override &&
+    exec_script("//build/check_return_value.py",
+                [
+                  "test",
+                  xcode_version,
+                  "-ge",
+                  "0730",
+                ],
+                "value") != 1) {
+  print(
+      "********************************************************************************")
+  print(
+      " WARNING: The Mac OS X SDK is incompatible with the version of Xcode. To fix,")
+  print(
+      "          either upgrade Xcode to the latest version or install the Mac OS X")
+  print(
+      "          $mac_sdk_min_build_override SDK. For more information, see https://crbug.com/620127.")
+  print()
+  print(" Current SDK Version:   $mac_sdk_version")
+  print(" Current Xcode Version: $xcode_version ($xcode_build)")
+  print(
+      "********************************************************************************")
+  assert(false, "SDK is incompatible with Xcode")
+}