Add child_window_open_by_dom_egtest.mm to EG2 target.

child_window_open_by_dom_egtest.mm file is already EG2 compatible. This
CL simply adds a file to an existing EG2 target.

Bug: 987646
Change-Id: I763129056ae5ca3f898a3a887eca8c59a0317747
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724821
Auto-Submit: Eugene But <eugenebut@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682190}

ios/chrome/browser/web/BUILD.gn

@@ -387,6 +387,7 @@ source_set("eg2_tests") {
   testonly = true
 
   sources = [
+    "child_window_open_by_dom_egtest.mm",
     "error_page_egtest.mm",
   ]