[iOS] Replace web::HttpServer in CookiesTestCase

Changes being made in crrev.com/c/2353092 are breaking these cookie
tests when using web::HttpServer. Since
net::http_server::EmbeddedTestServer supports setting cookie, these
tests can be updated to use the now recommended EmbeddedTestServer and
default handlers.

Bug: 891834
Change-Id: I08bdb620ad15b640c0beae22d712cca9fc5c7407
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363390Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799905}

ios/chrome/browser/net/BUILD.gn

@@ -83,7 +83,7 @@ source_set("eg2_tests") {
     "//ios/chrome/test/earl_grey:eg_test_support+eg2",
     "//ios/testing/earl_grey:eg_test_support+eg2",
     "//ios/third_party/earl_grey2:test_lib",
-    "//ios/web/public/test/http_server",
+    "//net:test_support",
     "//ui/base",
     "//url",
   ]