Add gclient_gn_args_file to DEPS (#10153)

Flutter uses its own DEPS file to checkout dart. To unblock
https://dart-review.googlesource.com/c/sdk/+/107837 Dart GN needs to not
fail to find build/config/gclient_args.gni even when flutter checks it
out. To fix this, this change just adds the same change to Flutter's
DEPS so that Flutter will generate the needed GN file for Dart. After
this change lands we can reland 107837.

DEPS

@@ -112,8 +112,20 @@ vars = {
 
   # Checkout Windows dependencies only if we are building on Windows.
   'download_windows_deps' : 'host_os == "win"',
+
+  # An LLVM backend needs LLVM binaries and headers. To avoid build time
+  # increases we can use prebuilts. We don't want to download this on every
+  # CQ/CI bot nor do we want the average Dart developer to incur that cost.
+  # So by default we will not download prebuilts. This varible is needed in
+  # the flutter engine to ensure that Dart gn has access to it as well.
+  "checkout_llvm": False,
 }
 
+gclient_gn_args_file = 'src/third_party/dart/build/config/gclient_args.gni'
+gclient_gn_args = [
+  'checkout_llvm'
+]
+
 # Only these hosts are allowed for dependencies in this DEPS file.
 # If you need to add a new host, contact chrome infrastructure team.
 allowed_hosts = [