[ios] Moves GetContentAndFragmentForUrl() into ios/net.

For EG2 tests, we'd like to use ios/web/public/test/http_server from the
test process, but without pulling in all of //ios/web.  Moving this
utility function allows http_server to depend only on //base, //net, and
//url.

BUG=930859

Change-Id: Ia1e35b04089a3bd2cf00ee0e7a663783ac2762a7
Reviewed-on: https://chromium-review.googlesource.com/c/1473616
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632352}

• ios/chrome/browser/web/BUILD.gn[diff]
• ios/chrome/browser/web/browsing_egtest.mm[diff]
• ios/chrome/browser/web/navigation_egtest.mm[diff]
• ios/chrome/browser/web/push_and_replace_state_navigation_egtest.mm[diff]
• ios/chrome/browser/web/window_open_by_dom_egtest.mm[diff]
• ios/net/BUILD.gn[diff]
• ios/net/url_test_util.cc[Added - diff]
• ios/net/url_test_util.h[Added - diff]
• ios/web/public/test/http_server/BUILD.gn[diff]
• ios/web/public/test/http_server/http_server.mm[diff]
• ios/web/public/test/url_test_util.h[diff]
• ios/web/test/url_test_util.mm[diff]