Feed V2: Add a feature flag in src/components/feed

Bug: 1119485
Change-Id: Ic85c4ae86c8bdc287e724f3bf403a7942aa1c337
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364023Reviewed-by: Cathy Li <chili@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799853}

components/feed/features.gni

@@ -4,4 +4,5 @@
 
 declare_args() {
   enable_feed_in_chrome = is_android
+  enable_feed_v2 = is_android
 }