Reland "views: Assert toolkit_views=true in BUILD.gn"

This is a reland of f93bc96d

Original change's description:
> views: Assert toolkit_views=true in BUILD.gn
>
> Now that Chromecast Linux enables this, all builds that link //ui/views
> are enabling this in GN. Add an assert so that accidental dependencies
> are diagnosed by "gn gen".
>
> Bug: 1021770
>
> Change-Id: I681645e97f69c22ee0410c1dba95319fddef7038
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898622
> Commit-Queue: Michael Spang <spang@chromium.org>
> Reviewed-by: Robert Liao <robliao@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#715524}

Bug: 1021770
TBR: robliao@chromium.org

Change-Id: I6ce35ae59265a1ccc866256d1730a8d07fb88496
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1934430Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721212}

ui/views/BUILD.gn

@@ -12,6 +12,8 @@ import("//ui/base/ui_features.gni")
 import("//ui/ozone/ozone.gni")
 import("//ui/views/features.gni")
 
+assert(toolkit_views)
+
 # Reset sources_assignment_filter for the BUILD.gn file to prevent
 # regression during the migration of Chromium away from the feature.
 # See build/no_sources_assignment_filter.md for more information.