Add error_page_egtest.mm to ios/chrome/browser/web:eg2_tests target.

This file is already EG2 compatible.

Bug: 922813
Change-Id: Ib5b072dd7d1a3973017c2ee490c5d7e42dce335d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643239Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666339}

ios/chrome/browser/web/BUILD.gn

@@ -381,7 +381,9 @@ source_set("eg2_tests") {
   ]
   testonly = true
 
-  sources = []
+  sources = [
+    "error_page_egtest.mm",
+  ]
 
   deps = [
     "//components/content_settings/core/common",