Move some feature flags to declare_args() so features can be disabled

BUG=

Review URL: https://codereview.chromium.org/959303002

Cr-Commit-Position: refs/heads/master@{#318668}

build/config/features.gni

@@ -49,6 +49,29 @@ declare_args() {
 
   # Enables proprietary codecs and demuxers; e.g. H264, MOV, AAC, and MP3.
   proprietary_codecs = false
+
+  enable_configuration_policy = true
+
+  # Enables support for background apps.
+  enable_background = !is_ios && !is_android
+
+  enable_captive_portal_detection = !is_android && !is_ios
+
+  # Enables use of the session service, which is enabled by default.
+  # Android stores them separately on the Java side.
+  enable_session_service = !is_android && !is_ios
+
+  enable_plugin_installation = is_win || is_mac
+
+  enable_app_list = !is_ios && !is_android
+
+  enable_supervised_users = !is_ios
+
+  enable_autofill_dialog = !is_ios && !(is_android && is_android_webview_build)
+
+  enable_google_now = !is_ios && !is_android
+
+  enable_one_click_signin = is_win || is_mac || (is_linux && !is_chromeos)
 }
 
 # Additional dependent variables -----------------------------------------------
@@ -113,11 +136,6 @@ if (is_android) {
   safe_browsing_mode = 1
 }
 
-enable_configuration_policy = true
-
-# Enables support for background apps.
-enable_background = !is_ios && !is_android
-
 enable_task_manager = !is_ios && !is_android
 
 use_cups = is_desktop_linux || is_mac
@@ -127,27 +145,14 @@ enable_themes = !is_android && !is_ios
 # TODO(scottmg) remove this when we've fixed printing.
 win_pdf_metafile_for_printing = true
 
-enable_captive_portal_detection = !is_android && !is_ios
-
-# Enables use of the session service, which is enabled by default.
-# Android stores them separately on the Java side.
-enable_session_service = !is_android && !is_ios
-
 # Whether we are using the rlz library or not.  Platforms like Android send
 # rlz codes for searches but do not use the library.
 enable_rlz = is_chrome_branded && (is_win || is_mac || is_ios || is_chromeos)
 
-enable_plugin_installation = is_win || is_mac
-
-enable_app_list = !is_ios && !is_android
 enable_settings_app = enable_app_list && !is_chromeos
 
-enable_supervised_users = !is_ios
-
 enable_service_discovery = enable_mdns || is_mac
 
-enable_autofill_dialog = !is_ios && !(is_android && is_android_webview_build)
-
 enable_wifi_bootstrapping = is_win || is_mac
 
 # Image loader extension is enabled on ChromeOS only.
@@ -155,10 +160,6 @@ enable_image_loader_extension = is_chromeos
 
 enable_remoting = !is_ios && !is_android
 
-enable_google_now = !is_ios && !is_android
-
-enable_one_click_signin = is_win || is_mac || (is_linux && !is_chromeos)
-
 # Chrome OS: whether to also build the upcoming version of
 # ChromeVox, which can then be enabled via a command-line switch.
 enable_chromevox_next = false