[experiences] Changed flutter_driver_enabled arg configuration

- Moved out flutter_driver_enabled arg declaration from
/session_shells/ermine/shell/BUILD
- Added flutter_driver_enabled arg to /bin/simple_browser/BUILD

Change-Id: I3da7fd97d2d1ac6ef13af17067aeff0ef54d65fb
diff --git a/bin/simple_browser/BUILD.gn b/bin/simple_browser/BUILD.gn
index 2756107..655e48b 100644
--- a/bin/simple_browser/BUILD.gn
+++ b/bin/simple_browser/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/config.gni")
 import("//build/fidl/fidl.gni")
 import("//build/testing/environments.gni")
+import("//build/testing/flutter_driver.gni")
 import("//topaz/runtime/dart/dart_fuchsia_test.gni")
 import("//topaz/runtime/dart/flutter_test.gni")
 import("//topaz/runtime/flutter_runner/flutter_app.gni")
diff --git a/session_shells/ermine/shell/BUILD.gn b/session_shells/ermine/shell/BUILD.gn
index 6b8c58c..2a42845 100644
--- a/session_shells/ermine/shell/BUILD.gn
+++ b/session_shells/ermine/shell/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/config.gni")
 import("//build/fidl/fidl.gni")
 import("//build/package.gni")
+import("//build/testing/flutter_driver.gni")
 import("//src/lib/icu/tools/static_icu_data.gni")
 import("//topaz/runtime/dart/flutter_test.gni")
 import("//topaz/runtime/flutter_runner/flutter_app.gni")
@@ -16,11 +17,6 @@
   ]
 }
 
-declare_args() {
-  # Enable flutter_driver builds. This is effective only on debug builds.
-  flutter_driver_enabled = false
-}
-
 tz_ids_path = "${target_gen_dir}/tz_ids.txt"
 
 flutter_app("ermine_app") {