Add GN restriction that //content isn't used on iOS

Similar to the restriction in //chrome/BUILD.gn. If you're trying to
use //content on iOS, you want to be using //ios/web instead. If you
need to use //content on other platforms and also need your feature to
run on iOS, it should be a layered component.

Change-Id: Ibe5f2cb9a3013d5a5160254ba6ee837f0e49378d
Reviewed-on: https://chromium-review.googlesource.com/c/1466983
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631618}

content/BUILD.gn

@@ -15,6 +15,8 @@ config("content_implementation") {
   ]
 }
 
+assert(!is_ios, "Chromium/iOS shouldn't use anything in //content")
+
 # When targets depend on, e.g. //content/public/browser, what happens? To
 # facilitate the complexity here, the "public" targets are groups that forward
 # to the right thing depending on the build mode. Say for additional