Move enable_app_list to a buildflag.

chrome/common/features.gni

Index: chrome/common/features.gni
diff --git a/chrome/common/features.gni b/chrome/common/features.gni
index cfd943792bc0ceb428ed7082cc5f6847b73e0cd4..a2f0e5047e59289672ef2aced0342f8db0de38ac 100644
--- a/chrome/common/features.gni
+++ b/chrome/common/features.gni
@@ -15,6 +15,8 @@ declare_args() {
   # through the environment.
   android_java_ui = is_android
 
+  enable_app_list = is_chromeos
+
   # Enables support for background apps.
   enable_background = !is_ios && !is_android && !is_chromecast
 
@@ -66,6 +68,7 @@ declare_args() {
 }
 
 chrome_grit_defines = [
+  "enable_app_list=$enable_app_list",
   "enable_background=$enable_background",
   "enable_google_now=$enable_google_now",
   "enable_hotwording=$enable_hotwording",