[iOS][Multiball] Add multi-window plist entries to eg2 test target.

The eg2 test target build of Chrome wasn't adding in the plist entries
for multiwindow when the multiwindow gn flags were enabled, causing
the tests to immediately crash if run with those flags.

This CL copies the logic in i/c/app/BUILD.gn for including those plist
fragments into the eg2 test .gni.

Test: Compiled and ran an eg2 sub-suite with the multiwindow flags; it
ran as expected (with some test failures, but no immediate crashes).

Bug: 1105529
Change-Id: Ieb2541104fb70e7036497242f1df07cd93f2595e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299344
Auto-Submit: Mark Cogan <marq@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789883}

ios/chrome/test/earl_grey2/chrome_ios_eg2_test.gni

@@ -33,6 +33,13 @@ template("chrome_ios_eg2_test_app_host") {
       if (ios_chrome_info_plist_additions != []) {
         info_plists += ios_chrome_info_plist_additions
       }
+      if (ios_enable_multi_window) {
+        info_plists +=
+            [ "//ios/chrome/app/resources/MultiWindowEnabled+Info.plist" ]
+      } else if (ios_enable_scene_startup) {
+        info_plists +=
+            [ "//ios/chrome/app/resources/MultiWindowDisabled+Info.plist" ]
+      }
       if (ios_chrome_info_plist_addition_targets != []) {
         if (!defined(deps)) {
           deps = []