Add ios_enable_prebuilt_widget_kit_extension flag

This flag will be used until the widget can be compiled
using the standard toolchain.

Change-Id: I0d7cc9187711be1d8743ea5c29093b9cefaf30f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514418
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823477}

ios/build/chrome_build.gni

@@ -14,6 +14,9 @@ declare_args() {
   # Enable iOS 14, WidgetKit extension.
   ios_enable_widget_kit_extension = false
 
+  # Enable iOS 14, WidgetKit extension (prebuilt).
+  ios_enable_prebuilt_widget_kit_extension = false
+
   # Enable share extension.
   ios_enable_share_extension = true
 
@@ -77,6 +80,8 @@ if (target_environment == "catalyst") {
   ios_enable_content_widget_extension = false
   ios_enable_search_widget_extension = false
   ios_enable_credential_provider_extension = false
+  ios_enable_widget_kit_extension = false
+  ios_enable_prebuilt_widget_kit_extension = false
 }
 
 # Configure whether breakpad support is enabled.