Document IdentityTestEnvironment's requirement of a task environment

After https://chromium-review.googlesource.com/c/chromium/src/+/1302175,
IdentityTestEnvironment now requires a properly set-up task environment.
The error that ensues at runtime if this is lacking is currently quite
cryptic. This CL adds documentation of the requirement and a transparent
runtime error for the case where the task environment is missing.

Bug: 798699
Change-Id: I7187b9d30f3072c2a0e697e7e325b01d238665bc
Reviewed-on: https://chromium-review.googlesource.com/c/1323073
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606417}

services/identity/public/cpp/identity_test_environment.cc

@@ -7,6 +7,7 @@
 #include "build/build_config.h"
 
 #include "base/run_loop.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "components/signin/core/browser/account_consistency_method.h"
 #include "components/signin/core/browser/test_signin_client.h"
 #include "components/sync_preferences/testing_pref_service_syncable.h"
@@ -145,6 +146,12 @@ IdentityTestEnvironment::IdentityTestEnvironment(
     std::unique_ptr<IdentityManagerDependenciesOwner> dependencies_owner,
     IdentityManager* identity_manager)
     : weak_ptr_factory_(this) {
+  DCHECK(base::ThreadTaskRunnerHandle::Get())
+      << "IdentityTestEnvironment requires a properly set up task environment. "
+         "If your test has an existing one, move it to be initialized before "
+         "IdentityTestEnvironment. Otherwise, use "
+         "base::test::ScopedTaskEnvironment.";
+
   if (dependencies_owner) {
     DCHECK(!(account_tracker_service || token_service || signin_manager ||
              gaia_cookie_manager_service || identity_manager));

services/identity/public/cpp/identity_test_environment.h

@@ -26,6 +26,10 @@ class IdentityManagerDependenciesOwner;
 // provides facilities for driving that IdentityManager. The IdentityManager
 // instance is brought up in an environment where the primary account is
 // not available; call MakePrimaryAccountAvailable() as needed.
+// NOTE: IdentityTestEnvironment requires that tests have a properly set up
+// task environment. If your test doesn't already have one, use a
+// base::test::ScopedTaskEnvironment instance variable to fulfill this
+// requirement.
 class IdentityTestEnvironment : public IdentityManager::DiagnosticsObserver {
 #if defined(OS_CHROMEOS)
   using SigninManagerForTest = FakeSigninManagerBase;