[Extensions] Re-enable an Identity API test.

GetAuthTokenFunctionPublicSessionTest.Whitelisted was disabled
for flakiness in 2018. I tested this on both release and debug
configurations of ChromeOS and it passed 3000 iterations.

Bug: 830052
Change-Id: Ibdca8b58eb9800946791c919f098dd16284e22bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209388Reviewed-by: Archana Simha <archanasimha@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770868}

chrome/browser/extensions/api/identity/identity_apitest.cc

@@ -2342,9 +2342,7 @@ IN_PROC_BROWSER_TEST_F(GetAuthTokenFunctionPublicSessionTest, NonWhitelisted) {
   EXPECT_FALSE(func->scope_ui_shown());
 }
 
-// TODO(crbug.com/830052): This test is flaky.
-IN_PROC_BROWSER_TEST_F(GetAuthTokenFunctionPublicSessionTest,
-                       DISABLED_Whitelisted) {
+IN_PROC_BROWSER_TEST_F(GetAuthTokenFunctionPublicSessionTest, Whitelisted) {
   // GetAuthToken() should return a token for whitelisted extensions.
   user_manager::ScopedUserManager user_manager_enabler(
       base::WrapUnique(user_manager_));