Pass path to gn binary to cronet_licenses.py when building for iOS.

components/cronet/ios/BUILD.gn

Index: components/cronet/ios/BUILD.gn
diff --git a/components/cronet/ios/BUILD.gn b/components/cronet/ios/BUILD.gn
index 38dfb99f79771606c85ce729862dbebcf1c2d239..f360d8b1f3c8425c726d8647c9a66dc85b61bb8c 100644
--- a/components/cronet/ios/BUILD.gn
+++ b/components/cronet/ios/BUILD.gn
@@ -146,6 +146,10 @@ if (additional_toolchains == [] || current_toolchain == default_toolchain) {
     _license_path = "$_package_dir/LICENSE"
 
     script = "//components/cronet/tools/cronet_licenses.py"
+    inputs = [
+      "//build/util/LASTCHANGE",
+      "//buildtools/$host_os/gn",
+    ]
     outputs = [
       _license_path,
     ]
@@ -153,6 +157,8 @@ if (additional_toolchains == [] || current_toolchain == default_toolchain) {
       "license",
       rebase_path(_license_path, root_build_dir),
       "--gn",
+      "--gn-path",
+      rebase_path("//buildtools/$host_os/gn", root_build_dir),
     ]
   }