[iOS] Don't create a TestUrlLoaderFactory for IdentityTestEnvironment.

None of the iOS tests need a TestUrlLoaderFactory, so we should not
create one by default.

Once FakeGaiaCookieManagerService is eliminated, tests will need to own
the TestUrlLoaderFactory themselves, so it's better not to have a
TestUrlLoaderFactory be created except when it will actually be used.

This is part of step 1a of the plan to eliminate FakeGCMS entirely:
https://docs.google.com/document/d/1t0ZtuV7h-znzdItFgBW0aKPscAwWXIBuNZnNlEGgi7g/edit

Bug: 907782
Change-Id: I420748e43f468858c33bd3d6c8b32f182e50a88f
Reviewed-on: https://chromium-review.googlesource.com/c/1382497
Commit-Queue: Lowell Manners <lowell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619640}

ios/chrome/browser/signin/identity_test_environment_chrome_browser_state_adaptor.h

@@ -32,7 +32,7 @@ class IdentityTestEnvironmentChromeBrowserStateAdaptor {
   static std::unique_ptr<TestChromeBrowserState>
   CreateChromeBrowserStateForIdentityTestEnvironment(
       const TestChromeBrowserState::TestingFactories& input_factories,
-      bool create_fake_url_loader_factory_for_cookie_requests = true);
+      bool create_fake_url_loader_factory_for_cookie_requests = false);
 
   // Creates and returns a TestChromeBrowserState that has been configured with
   // the given |builder|.
@@ -40,14 +40,14 @@ class IdentityTestEnvironmentChromeBrowserStateAdaptor {
   static std::unique_ptr<TestChromeBrowserState>
   CreateChromeBrowserStateForIdentityTestEnvironment(
       TestChromeBrowserState::Builder& builder,
-      bool create_fake_url_loader_factory_for_cookie_requests = true);
+      bool create_fake_url_loader_factory_for_cookie_requests = false);
 
   // Sets the testing factories that identity::IdentityTestEnvironment
   // requires explicitly on a Profile that is passed to it.
   // See the above variant for comments on common parameters.
   static void SetIdentityTestEnvironmentFactoriesOnBrowserContext(
       TestChromeBrowserState* browser_state,
-      bool create_fake_url_loader_factory_for_cookie_requests = true);
+      bool create_fake_url_loader_factory_for_cookie_requests = false);
 
   // Appends the set of testing factories that identity::IdentityTestEnvironment
   // requires to |factories_to_append_to|, which should be the set of testing