Add checkout_nacl gclient flag (default true)

See https://groups.google.com/a/chromium.org/d/msg/chromium-dev/tO8kIrDtQc8/bCRO-UXxBQAJ for discussion.

Bug: 756688
Change-Id: I80f0526b278a813b398f6f81baacf36552da1792
Reviewed-on: https://chromium-review.googlesource.com/681854
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#504598}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f036ddd72e5ca3de3c819e1d7c8163c2de8e60f2

.gitignore

@@ -2,6 +2,7 @@
 # mirror in DEPS. Without it, a lot is wiped and re-downloaded for each sync.
 /android/bin
 /android/binary_size/apks/**/*.apk
+/config/gclient_args.gni
 /Debug
 /Debug_x64
 /goma

config/features.gni

@@ -16,6 +16,7 @@
 
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
+import("//build/config/gclient_args.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
 }
@@ -24,10 +25,15 @@ declare_args() {
   # Enables Native Client support.
   # Temporarily disable nacl on arm64 linux to get rid of compilation errors.
   # TODO(mcgrathr): When mipsel-nacl-clang is available, drop the exclusion.
-  enable_nacl = !is_ios && !is_android && !is_fuchsia && !is_chromecast &&
+  enable_nacl = checkout_nacl &&
+                !is_ios && !is_android && !is_fuchsia && !is_chromecast &&
                 current_cpu != "mipsel" && current_cpu != "mips64el" &&
                 !(is_linux && target_cpu == "arm64")
+}
 
+assert(!enable_nacl || checkout_nacl)
+
+declare_args() {
   # Non-SFI is not yet supported on mipsel
   enable_nacl_nonsfi = current_cpu != "mipsel" && current_cpu != "mips64el"