[ios] expose enableMockAuthentication for EG2 tests.

In EG2 a test that calls disableMockAuthentication must
re-eable on tearDown, or the app_interface can crash from
mismatch enable/disable.

Change-Id: I06c0a2feebb43cf1bcaa9b639a7a29deafa03094
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1953731Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723300}

ios/chrome/test/earl_grey/chrome_test_case.h

@@ -37,6 +37,9 @@ class EmbeddedTestServer;
 // end of the test. This shall only be called once per test.
 - (void)disableMockAuthentication;
 
+// Turns mock authentication back on.
+- (void)enableMockAuthentication;
+
 // Stops the HTTP server. It will be re-started at the end of the test. This
 // should only be called when the HTTP server is running. This shall only be
 // called once per test.

ios/chrome/test/earl_grey/chrome_test_case.mm

@@ -307,6 +307,13 @@ GREY_STUB_CLASS_IN_APP_MAIN_QUEUE(ChromeTestCaseAppInterface)
   _isMockAuthenticationDisabled = YES;
 }
 
+- (void)enableMockAuthentication {
+  // Enforce that enableMockAuthentication can only be called once.
+  DCHECK(_isMockAuthenticationDisabled);
+  [[self class] enableMockAuthentication];
+  _isMockAuthenticationDisabled = NO;
+}
+
 - (void)stopHTTPServer {
   // Enforce that the HTTP server can only be stopped once per test. It should
   // not be stopped if it is not running.